One day, after getting lost at the train station, Eugene accidentally ends up in the ghostly Underworld that coexists alongside the world of the living. He's quickly assaulted by monsters, only to be rescued by a ghost known as "Boss". Boss is revealed to hold an office in the underworld, where he and his underlings—River, Joan, Hayden, and Sean—right wrongs and pay back kindnesses to sort out their karma, in order to get closer to crossing to the Great Beyond. As Boss saved Eugene's life, he informs the boy that he is in debt to him and can pay him back by helping the office with three jobs. Over the next several nights, Eugene accompanies the ghosts on trips to help mortals who are suffering because of spirits; along the way, depending on the player's choices, he can learn important life lessons about self-worth and friendship.
The game has multiple endings, dependent on what dialogue options or actions the player picks for Eugene to take.
In June of 2022, a sequel called Charlie In Underworld was released that takes place several years after this game and follows Charlie who has lost their memories after being unsealed from Joan's cane.
Underworld Office contains examples of:
- The Alcoholic: Jack, the final mortal suffering from nightmares, is an infamous alcoholic who's nasty to his neighbors.
- All Just a Dream: After seeing the worst endings, the game, via Boss, admonishes the player to pick options you won't regret. Then Eugene awakens in a fright and says he had a worse nightmare than usual.
- And Then John Was a Zombie: One of the bad endings involves Eugene, himself, turning into a monster while in his spirit form. He is then exorcised by Boss.
- Anger Born of Worry: Boss gets enraged at Eugene when he sleeps early to visit the office, and effectively fires him. This was because he was very worried over the fact that Eugene was prioritizing the world of the dead over that of the living.
- Bittersweet Ending: The ending of the route in the final chapter where you decide to do things Charlie's way rather than Boss's way, but you save Jack from being murdered by Charlie. The situation is resolved, with Charlie exorcised, Jack arrested, and the children safe. But because he doesn't agree with Eugene's choice, Boss respectfully cuts ties with him and only promises that they will meet again one day, presumably when Eugene himself is dead.
- Cosmetic Award: The achievements in the game, called "titles", are all different outfits, poses, and filters you can apply to Eugene's character profile. One example is the Intern title, which changes him to look like his ghostly self. Befriending the ghosts at the office similarly unlocks titles that put Eugene in the clothes that they wore when they were alive.
- Dark and Troubled Past: The ghosts have them; River is shown having a lot of anger at the world, which is implied to be related to how she died, while Joan is revealed to have participated in a war, potentially the Korean War, and has a personal nightmare about her experiences. Boss, the oldest of the ghosts to the point that he's wearing traditional clothing, is noted to have a lot of misdeeds to pay back, and his silhouette is completely black even now. Even Charlie mentions how everyone ignored her when she was alive, something which apparently contributed to her death.
- Death of a Child: Hayden, one of the ghosts, is a very young child—a fact that Eugene finds horrifying. It's also possible to witness a scenario where Eugene doesn't interfere in the final chapter, and the two children from the park are later found dead by the police.
- Downer Ending: The ending during Charlie's chapter if you get caught and murdered by Jack counts, as well as what happens if you decide not to do anything about Jack later. Susan's children are murdered, Luke gives up trying to befriend Eugene, and he ends up even worse than before. If Eugene decides to go through with taking revenge on Jack for what he's done, yet another ending has Eugene murdering Jack and turning into a monster, with a sorrowful Boss then having to trap him in his fan like the other monsters. The worst endings, however, are handwaved away as being All Just a Dream.
- Humans Are the Real Monsters: Ghosts can become monsters from wracking up too much bad karma. In the case of the final job he does with Boss, however, Eugene comes to the conclusion that the ghost isn't the monster—the mortal who killed her in the first place is.
- Not Quite the Right Thing: Some of the jobs have choices like this. For example, in Luke's chapter, Eugene can choose to wake him up and prevent him from meeting Kitty to protect him from Kitty potentially harming him. But this actually prevents Luke from learning that his kitty loved him and didn't blame him for its death; Hayden specifically words it as Eugene tried and didn't get it right. Similarly, empathizing with Sean too much will only reinforce his self-hating behavior and lead him to spiral into a monstrous emotional breakdown.
- "Not So Different" Remark:
- In the route where Eugene lets Charlie escape, he can admit it's because he saw himself in the way Charlie kept casually belittling her own worth.
- In some routes, if Eugene kills Jack and becomes a monster, but maintains that he felt there was no other way, Boss will recruit him rather than capture him. This is because he believes Eugene did what he felt he had to even if it made him into a monster—which he claims is also why he became a dark ghost.
- Our Ghosts Are Different: Ghosts can be alive or dead, with Eugene frequently leaving his body in order to work with the other ghosts. Living ghosts still have color, though the colors are somewhat reversed from in life, while dead ghosts are black and white. The more misdeeds a ghost has to pay back, the blacker their silhouette is—and if you have too much bad karma against you, you turn into an outright monster.
- Parental Abandonment: Despite parents and their relationship with their children making up about half of the game's subplots, Eugene's parents are nowhere to be seen and he notes that he has no one to hang out with and no one waiting for him to come home. It's not clear if they abandoned him or are simply neglectful.
- Rule of Three: Eugene has to help with three jobs before his debt is fulfilled. Although he does handle a more ghosts than that, this is because he was kicked out of the office during his third ghost encounter, and so the fourth is technically the third job.
- Stealth Hi/Bye: Boss often appears very suddenly, intruding on conversations and startling everyone involved.
