The episode begins with a few lines of text explaining that the starship Hermes has been destroyed by a synthetic, highly corrosive micro-organism. An escape pod from the ship carries a lone survivor, Talia Garrett. The pod and the woman are picked up by Red Dwarf, but is also revealed that the microbe has also been brought on board when part of the empty escape pod dissolves away.
Now some time into their prison sentence, the Dwarfers have been put on probation for good behaviour. Arnold Rimmer is attending to Captain Hollister as he recovers from Yellow Fever. Hollister notices that Rimmer has tried to slip in a form pardoning him from all crimes. Rimmer explains his ambition to become an officer someday, perhaps even a Captain. Hollister tells him he is not officer material and dismisses him as Talia enters. It seems that the Captain and Talia know each other from the past. Rimmer, disgusted at the Captain's success with woman and his own failure, leaves. In the corridor he attempts to steal some chocolate from the vending machine, only for the AI of the machine to berate him.
Meanwhile, back in The Tank, Kryten tells Lister on how he is changing Miss Kochanski's calendar. Lister realises what is going on and instructs Kryten secretly on how to behave. Kochanski returns to her cell and discovers Kryten has attempted to celebrate her period with a banner that reads 'Have A Fantastic Period'.
Later, in Lister and Rimmer's cell, they discover that Kryten has hidden Baxter's illegal moonshine in their cell, just before Holly informs them of a cell inspection. With the water tank full they have no choice but to drink the hooch. When Baxter finds out, believing his hooch was stolen by Rimmer and Lister, he threatens to finish off the two, now in sickbay, sleeping off the stomach pump. The two decide they need to escape. Kryten and Kochanski pretend to be ill to land in sickbay, while the Cat disguises himself as a nurse. As the Dwarfers escape they find that the microbe from the Hermes is now eating away at Red Dwarf.
Kryten and Kochanski devise a plan which involves entering a mirror universe where everything is opposite; negative becomes positive and a virus becomes an antidote. Kryten builds a prism laser and directs it at a mirror to create a dimensional gateway. Rimmer crosses over, only for the prism to break and trap him in the mirror universe. He realises he is not a failure in this universe but is instead Captain, and berates the alternative Hollister, now a 2nd Technician, for being useless. When Talia comes in, Rimmer, thinking she is his lover, snogs her, only to be told she is his spiritual advisor. Rimmer quickly goes to the science lab to talk to the professor, the alternative Cat, and gets the antidote
When Rimmer returns to his universe he finds the other Dwarfers have repaired the prism and followed him into the mirror universe. By now Red Dwarf is a flaming inferno and falling apart. Rimmer tries to return to the mirror universe, but the microbe has eaten away the laser. He tries to create the antidote, but the dispensing machine reveals that the formula from the mirror universe is now the formula for the virus, having reverted into its mirror opposite. After exchanging insults, the machine sends a can flying towards Rimmer, knocking him unconscious.
In what may be either a hallucination or reality, the Grim Reaper, played by series director Ed Bye, comes to claim Rimmer. Rimmer kicks Death in the crotch and runs off down the flaming corridor, claiming that "only the good die young" as Death winces "That's never happened before..." The episode ends with an ominous "The End" caption, but is soon refuted with its sudden erasure and replacement with "THE SMEG IT IS".