Former UNIT Captain Mike Yates arrives at the modern-day UNIT offices to make a report on an alien artifact recently unearthed and given to him by Jo Grant: a playing card, the Jack of Diamonds.
Strange things are afoot in London. Despite the July heat, localized cold spells are occurring throughout the city: a frosted lawn here, an Underground tunnel filled with snow there, all culminating in the flash freezing of half of Hyde Park in mere seconds, killing everybody present.
The UNIT crew find and question a survivor, a woman who says she was part of a street magician's vanishing act, which transported her elsewhere. The Doctor and the Brigadier dismiss this magician as irrelevant, preferring to follow up on some strange energy readings the Doctor's instruments have detected, but Mike and Jo decide to investigate him on their own.
They find the magician, Diamond Jack, performing magic tricks downtown for a huge crowd. Mike ducks out to be chastised via radio by the Brig for going AWOL, and when he returns, he finds that Jo has volunteered to be part of Jack's act and vanished. When questioned, Jack seems confused, then clues Mike in to Jo's location before vanishing himself, pulling heat out if the air as he does so.
Mike finds Jo in Jack's barely-used house on the outskirts of the city and radios for the Brig. He and the Doctor arrive and explain that Jack is, indeed, behind all the strange cold snaps. Jack shows up and explains that since he was a child, he could drain energy from things - even the very air - and use it to manipulate matter. He tries to give a full account of his childhood, but in reality is simply recalling that of Jo, with whom he is psychically linked. The Doctor pulls out a strange device he found in a spaceship nearby and accidentally restores Jack's memories of being an alien criminal.
He cacklingly vanishes as the Brig tries to shoot him. For once the Doctor spares him the gun lecture and the group returns to the spaceship to find more information about Jack. They discover that he is a mass murderer who was sentenced to exile to Earth in a human body with his memories stored in his real body. Said real body, a horrific insectoid mouth monster, tracks down Jack using the ship's computers and sets off to reunite body and mind. The UNIT crew, seeing that Jack is at the Tower Bridge, rush to beat it there.
The classic showdown at the top of the bridge culminates in Jo tricking Jack into thinking that his arriving body is a member of his species' police and killing it. The stored memories start spilling into his human mind, which doesn't have the capacity for them. Dying, he asks Jo to let him do one more trick. He hands her a Jack of Diamonds and begins annihilating her mind, intending to use her as storage. Mike, despite the risk to Jo, shoots Jack dead.
In the end, Jo is fine after a little recuperation, though she has no memory of the adventure until years later, when she finds the playing card Jack gave her. It turns out that it was a memory storage unit - Jack had never intended to permanently destroy Jo's mind after all.
Tropes:
- All Love Is Unrequited: A theme of this story is Mike's Doomed by Canon love for Jo.
- Energy Absorption: Jack uses this to power his abilities, at one point blacking out all of London.
- Five Rounds Rapid: The Brigadier's attempt to shoot Jack is predictably useless.
- Foil: An alien exiled to Earth by his people without part of his memories for capital crimes? Sounds familiar somehow...
- Lower-Deck Episode: Like most Companion Chronicles, the Doctor spends most of his time offscreen while the normally secondary characters do their own thing.
- Pet the Dog: Jack stores Jo's memories before trying to wipe her mind.
