At 221B Baker Street (The Doctor’s residence), Jago and Litefoot are preparing to meet with their prospective biographer. Jago is overcome with excitement, while Litefoot is still withholding reservations. The door opens and a middle-aged man enters introducing himself as Carruthers Summerton. Summerton immediately gushes over meeting the Infernal Investigators in person. Litefoot reminds Summerton that he is conducting an interview and asks what exactly he wants for his book. Summerton wishes to conduct an interview with both investigators privately as well as together, plus he is also gathering any paraphernalia the Infernal Investigators may come across in their adventures so any donations would be welcome. But what Summerton really wants is to accompany Jago and Litefoot on a case to see them both at work. The only problem is that the Infernal Investigators are currently not working any cases, to which Summerton suggests they let Inspector Quick know that they are available…after all, Quick is a main supporting character in their cases…
In the grounds of a townhouse, young couple Billy and Nancy are enjoying some intimacy when they hear pained groans and screaming coming from the bushes. They discover a naked man emerge from the bushes groaning angrily. Billy urges Nancy to climb a nearby tree to keep her away from the indecent individual…
The next day, Jago and Litefoot have been summoned by Inspector Quick to the police station. Quick mentions that their ‘new agent’ said the Infernal Investigators are willing to take on any mission, much to their collective chagrin. As if on cue, Summerton arrives and begins to take notes as Quick gives the investigators their assignment. They are to investigate the murder of a Mr. McCallum, a manservant to a gentleman called Sir Hartley Harecourt. He explains that McCallum was murdered in a room locked from the inside and that a gasogene (soda siphon) was stolen afterwards. Summerton is eager to begin investigating and states that they should visit Hartley’s Townhouse immediately.
The Infernal Investigators and Summerton arrive at Hartley’s townhouse. His servant Nancy summons Hartley and moments later, a small train chuffs into the foyer with a man gleefully riding on top of it. The man introduces himself as Hartley and shows off his invention – the house train. The Infernal Investigators introduce themselves and state they have come to investigate the murder of McCallum. Strangely, Harecourt doesn’t seem to immediately recall his own manservant’s death, but nonetheless explains the circumstances. Hartley had tried to access the locked room but was unable to do so. He tried to summon McCallum to unlock the door but received no response. Rather than search for his manservant, Hartley began to write a letter of his dismissal. Eventually, Nancy suggested that McCallum and the locked room may be connected, so Hartley summoned his groundskeeper to break down the door and, in the room, they found McCallum dead. Litefoot asks to see the crime scene.
Hartley shows them into the room and explains he found McCallum slumped over his best chair but didn’t immediately assume he was dead as he assumed that McCallum was hungover. But aside from McCallum’s resting form, Hartley noticed that his ‘family heirloom’ gasogene was stolen. The Infernal Investigators decide to leave to conduct their inquiries, but not before Hartley offers Jago a cigar from one of his inventions – the cigar-o-matic, which launches a cigar at him at considerable speed!
That evening, The Infernal Investigators and Summerton arrive at the Red Tavern. The two men are slightly unnerved by the fact that not only does Summerton know who Ellie is, but he also knows their preferred table and choices of alcohol. Litefoot wishes to begin his line of inquiry by investigating the locked room mystery to gain a further insight into how the killer could’ve got inside. Jago has an alternative theory…the killer was in the gasogene! It could be a murderous ghost, a genie or an alien consciousness made of smoke! Both men begin to argue until they notice the entire pub has fallen silent…Litefoot isn’t completely convinced and decides to return to Hartley’s house the next day and begin a thorough search of the crime scene. While Jago plans to visit a gasogene shop to follow up on the missing gasogene lead.
The next day, Litefoot is investigating the crime scene at Hartley’s house. Summerton notices a necktie on the floor with the mathematical sigma symbol on it, plus a tag attached to it with the letters J.C. printed on it. Litefoot deduces that the house has a hidden room in the study by calculating the accessible square feet measurements on the ground floor. He theorises that this hidden room could have an entrance outside the house, hence how the killer managed to access the locked room! He taps the walls and discovers the candlestick above the fireplace opens a door where the fireplace should’ve been…
Litefoot and Summerton discover the secret room is in fact a laboratory! Hartley and Nancy arrive in the laboratory, startling the investigators. Hartley asks the two if they’ve met before, to which Litefoot finds disturbing considering they only just met yesterday. Litefoot explains that this is how the murderer managed to gain entry to the locked room and escape as Litefoot predicted, there is a door leading out of the house. When Litefoot asks about the laboratory’s purpose, Hartley explains that he is working on experiments to benefit mankind but doesn’t elaborate how. Litefoot shows him the necktie Summerton discovered, the sight of the tie makes Hartley unusually nervous and asks if he can have it (even asking to swap it for one of his many ties), but Litefoot refuses as it is evidence. Summerton prepares to leave as it is time for him to follow Jago and his inquiries into the missing gasogene…
Jago and Summerton arrive at Grayson’s Gasogenes. Jago has invited the owner of the gasogene shop to be interviewed. The owner, Grayson, arrives and confirms that they manufacture the gasogene that went missing from the scene of the crime. After confirming how a gasogene works, Jago asks the most pressing question…could a ghost or gas creature survive inside one? Grayson believes Jago to be mentally insane and orders him out of the shop.
That evening, at the Red Tavern. Litefoot relies the fruits of his investigation to Jago. After looking into the sigma symbol on the necktie discovered at the scene of the crime, Litefoot discovered it is also the emblem of the Oxford Scientific Society. He made an inquiry with the society to see if anyone had the initials J.C. in their ranks and discovered one individual – Jasper Cornish. In finality, Litefoot plans to visit the society tomorrow to meet with Cornish. Jago doesn’t have much to offer from his investigation until Ellie arrives with a message. Jago gets up and Ellie whispers that the missing gasogene she was asked to keep an eye out for has been spotted in the window of a pawnbrokers. Jago is thankful but tells her to keep it to herself and not a word to Litefoot. Jago returns to the table and proudly tells the group one of his leads has borne fruit…
Meanwhile, Jasper Cornish is writing a letter to Hartley telling him that they need to meet urgently. His cat Dorothy starts to hiss at the door. But Cornish reassures her that its just the wind…until a knock sounds at the door. A man with a coarse Scottish accent asks for help on the other side, stating that he needs Cornish’s formula. Cornish pensively opens the door and comes face to face with an incredibly hairy looking beast. Cornish fearfully asks what he is. The beast simply states that he should know…he made him…
The next morning, Jago arrives at the Pawnbrokers to follow his gasogene lead. He tells the woman behind the counter that he is interested in purchasing the gasogene in the window. They both start haggling prices until they both settle on 8 shillings. She hands him a receipt and bids him good day. Jago leaves the shop with the gasogene in hand and looks at the receipt and realises that the woman who served him has the same name as their victim…McCallum…
Litefoot and Summerton arrives at Cornish’s residence. Litefoot knocks on the door to no response, but find that its open anyway. They enter and find a large, hairy man dead on the floor. Litefoot takes a closer look at the largely built body and ragged clothes and deduces that this man can’t be Cornish…but then who is he? They turn and discover another body on the ground dead, this person Litefoot positively identifies as Cornish. With no murder weapon at the scene of the crime, there’s no discernible way to know how both men died. Litefoot decides that both men need to be taken to his mortuary to carry out an autopsy.
At the pawnbrokers, the woman who served Jago is gleefully counting the 8 shillings she received from Jago’s transaction. She enters the shop’s backroom and tells her brother Alastair that they have nearly enough money to let a doctor look into his abnormal condition. Alastair states that no doctor can help him now…he knows a scientist that can help him and he’s head out tonight to meet with him…
At the mortuary, Litefoot begins his operation on the large hairy man to discern the cause of death as Summerton watches on. Litefoot had already ascertained the cause of death for Cornish as heart failure. Litefoot opens the hairy man’s chest and makes a startling discovery…there are no internal organs. Litefoot recalls looking at some of Cornish’s paperwork and discovered that he was working on a formula called the Viserpation formula. Believing this to be key to the mystery, Litefoot and Cornish decide to investigate this mystery formula. Summerton recalls seeing the exact same words in Hartley’s lab, so they both decide to start their investigation there…
At Hartley’s home, Litefoot and Summerton await Hartley’s return as he is out of the house. Litefoot decides not to wait any longer and opens the secret lab to continue his investigation. He is surprised to see Summerton was correct, Hartley has the words viserpation formula written down on paperwork in the lab, confirming his connection to Cornish. They find several scientific papers scattered around the lab. One paper that catches Litefoot’s eye is a paper called the Speciation formula written by a Doctor H. Kindred. Litefoot remembers that name cropping up in Cornish’s papers as well. Litefoot swears Nancy to secrecy as he gathers up the scientific papers and leaves with Summerton.
Meanwhile at the Red Tavern. Jago is looking over the gasogene he bought from the pawnbrokers. Unable to discern anything supernatural or extra-terrestrial from it, he decides to visit Hartley’s house to return it.
Elswhere, Litefoot and Summerton visit the house of Doctor H. Kindred. They knock on the door and are greeted by a woman. Summerton, assuming the woman to be a servant, orders her to hurry along to fetch Doctor Kindred…only for the woman to inform them that she is in fact Doctor Kindred, much to Summerton’s disbelief. Litefoot politely introduces himself and Summerton and asks if she can explain her Speciation formula. Kindred allows Litefoot in to talk but slams the door in Summerton’s face for his sexist remarks.
Jago arrives at Hartley’s house and presents Hartley with the missing gasogene. Hartley is overjoyed to be reunited with it and invites Jago inside to a celebratory drink.
At Doctor Kindred’s home, Kindred explains the Speciation serum. It merely directs the course of speciation or the evolutionary process of all species. Her plan was to try and direct evolution towards a much steadier path of progress, unfortunately there were several complications as well as breakthroughs. Despite everything, Hartley generously sponsored the experiments. Litefoot mentions Cornish, but Kindred denies working with him, which is odd considering Hartley had both Cornish and Kindred in his notes as colleagues. A rapid knock on the door begins to sound followed by a coarse Scottish voice calling for Kindred. Litefoot tells her not to answer as he tells her that Cornish was recently found dead and he has reason to suspect that her life may be in danger as well. With that in mind, Kindred leads Litefoot to the garden to escape from the mystery man.
In the garden, Litefoot and Kindred jump the garden fence and reach the main road. They hear wood splintering as they see a large hairy man like the one discovered in Cornish’s home give chase after them, except this one is much larger and hairier. Litefoot flags down a cab with Summerton inside. They quickly scramble inside and dash off down the road. Kindred demands to know what is going on, so Litefoot hands her Hartley’s scientific papers to see if she can discern what they mean.
At Hartley’s house, Jago and Hartley are enjoying a few celebratory drinks. Hartley mentions how he never had much fondness for McCallum as he was a lush and a thief. He lets slip that it is the reason why he had no moral compunction when it came to using him in the testing phase of his experiment…the potentiation potion…
In the cab, Kindred reads through the scientific papers and finds that Hartley had taken excerpts of her Speciation formula and applied them to his experiments, as well as Cornish’s Viserpation formula, which she explains can split a living organism into two like an organism’s cells. Litefoot deduces that he is trying to combine the two formulas into a single potion…
At Hartley’s house, Hartley explains to Jago that he used to be a wide-eyed scientist hoping to do good with his knowledge. However, his research and theories got him kicked out of his institution. Once his family passed away, he used his fortune to fund other scientists as a benefactor. But he had graciously funded two scientists, Kindred and her Speciation formula, and Cornish with his Viserpation formula. His endgame is to fund them enough to make breakthroughs in their experiments and then use their research to perfect his own formula – the potentiation potion. With his potion, Hartley can rapidly accelerate the human evolutionary process in an instant over what nature would take thousands of years to produce. Hartley gleefully reveals that he tested the potion on McCallum in secret so that his knowing wouldn’t affect the outcome…by hiding it in the missing gasogene that he drunk his alcohol from!
Taking Litefoot’s theory into account, Kindred theorises that the combined potion would split the subject into two forms, the original subject and a highly evolved counterpart. Litefoot deduces that Hartley must have created a possible potion and used the test potion on McCallum, which in turn would’ve split him in two, which means that the dead body of McCallum they discovered at the crime scene would be missing his internal organs while the real McCallum is roaming about as the horrifying monster! The hairy beast at Cornish’s home was McCallum split apart after the initial effects of the potion seemingly killed him. When McCallum seemingly died after his scuffle with Cornish, he split apart taking his intact organs into his newer form and leaving the corpse an empty husk. Right now, the beast chasing after them has evolved from that death! Litefoot states that they need to get to Hartley’s home immediately and he has a plan!
Hartley gleefully explains that he used the gasogene’s two chambers to combine the two formulas to create his potentiation potion. Knowing McCallum would try to steal and drink some of his alcohol, Hartley purposefully left the gasogene on the drinks cabinet for McCallum to discover and eventually drink instinctively. If it failed, it would be no loss to Hartley as he would’ve done away with his servant, but miraculously it worked and the newly evolved McCallum snatched the gasogene and ran off into the streets of London, leaving behind his original dead body at the crime scene. Jago wearily asks if Hartley made sure to have not cross-contaminated his drink with the gasogene, after all he did serve his whiskey from it…to which Hartley denies as he cheerfully reveals that Jago is to be his newest test subject! Jago suddenly begins to feel faint as the potion (and the added sedatives Hartley added to his ice cubes to make sure Jago didn’t escape) begin to take effect. Hartley hears a knock on the door and Litefoot enters the room. Jago tries to explain to Litefoot the perilous situation he’s been made a part of, but the sedatives begin to take hold as he falls asleep. Hartley locks the door and tells Litefoot that he’s just in time to bear witness to humankind’s next step in evolution and nothing will stop him!
Unbeknownst to Hartley, Kindred and Summerton are in his laboratory working on an antidote. Unfortunately, she is unable to read Hartley’s writing or make sense of Cornish’s formula, so she focuses on creating a drug that will counteract the speciation side of the potion and pray that he hasn’t introduced something into the potion that can completely counteract her antidote…
In the drawing room, Hartley gleefully admits to Litefoot that he is not truly a scientist at all! More of a dabbler who has no true idea of what he’s doing! Suddenly Jago groans in pain, his voice becomes slightly distorted as he begins to transform…
In the laboratory, Kindred is finally able to create an antidote and orders Summerton to fetch a syringe.
Jago begins to growl animalistically as the transform begins to take hold. Kindred tries to open the door and declares she has the antidote. Unfortunately for Litefoot and Hartley…Jago is between them and the door. As if things couldn’t get any worse, McCallum’s hairy form jumps through the window into the room. Both monsters size each other up, but quickly turn back to Litefoot and Hartley. Suddenly, Kindred and Summerton smash their way through the door on the back of Hartley’s house train. They throw large quantities of the antidote to Litefoot and Hartley, but the monsters are getting restless and won’t let them near. Hartley realises he can use the cigar-o-matic to fire the syringes at the monsters. With two well placed shots, Litefoot and Hartley manage to administer the antidotes.
Later, Hartley forces a now human McCallum to apologise for causing all the commotion, even making him apologise for Hartley’s own faults. With all the monstrous business out of the way, Litefoot, Summerton and a now human Jago prepare to leave.
Outside, Summerton states that he has had a wonderful time following the investigators around on his first case and asks when they’ll be on a new one. Litefoot acts dismissive and tries to see off the journalist. Getting the hint that he’s not welcome, Summerton quickly leaves and bids the gentlemen goodbye. Once Summerton is out of earshot, both men agree that they found Summerton to be annoying and that they work best when it’s just the two of them on the case.
