Sarah Emmins, her husband Stuart Gorey and her research assistant Ian Bird have been working at the fifty-seventh research station on Fraternity. The dangerous chemical storms prevent them from gathering samples except during brief intervals in the storm.
One day after Ian returns to the station after collecting samples, he and the other two are surprised to see someone else return to the station, and even more surprised when he takes off his bio-suit to reveal that he is identical to Ian.
Sarah and the others quarantine the duplicate and take some samples from him, but they have no idea what he is. He is just as confused as the others. The head station shuts down the project for now and demands answers from the fifty-seventh.
Two experts arrive, and the head station orders that Sarah and her team cooperate. When the experts, who introduce themselves as the Doctor and Nyssa, arrive, there are some awkward moments but the Doctor assures them that he and Nyssa have dealt with difficult problems before. He proposes that they interview each of the station members, including the duplicate, separately.
They discover that Sarah has accepted the duplicate, Stuart is suspicious that it's part of some plot, and Ian believes some of Stuart's suggestions. The duplicate is concerned about his rights, as he has been experimented on, and begins to feel he is unique and that he should be an ambassador for whatever intelligent life may be on the planet.
Later, as the Doctor, Nyssa and Bird are in the recreation area, they hear an alarm. They find that Stuart has been attacked by the duplicate, who has taken Sarah and has Stuart's gun. They deduce that the duplicate has taken her to the collection room, but when they get there, they find he has changed the lock. Nyssa suggests they use the video link to let them see inside the room.
Inside the collection room, Sarah is tied to a chair. The duplicate seems to have anticipated that they would monitor him, so the Doctor tries to talk to him via the link. The duplicate makes his demands: he wants to be treated like an ambassador, he wants to be taken to the head station, and he wants to have a say in the project. When the Doctor informs him that they can't let him dictate terms, the duplicate shoots the video camera, and they lose the picture.
They head for the collection room, afraid that the duplicate will open the outside door, exposing Sarah to the chemical storms. They get the door open, and the Doctor enters, reasoning that he would be able to survive the storms longer than the others.
His attempts to placate the duplicate fail, however, and the duplicate opens the doors. He shoots the Doctor, and when Stuart and Ian enter, he shoots Stuart as well. Ian grabs Sarah, chair and all, and takes her from the room. The Doctor, shot in the leg, tries to rescue Stuart, but it's too late. He tells the others that he doesn't know what happened to the duplicate.
On their way back to the TARDIS, the Doctor and Nyssa lament their failure. The Doctor isn't consoled to be reminded that he tried his best, because trying his best and failing offends his "sense of optimism". He promises Nyssa a vacation.
- The stories in Short Trips: Seven Deadly Sins are based on the Christian concept of seven deadly sins. This story's theme is pride.