While cruising through space in Starbug, the Dwarfers find that they gone off course from their path back to Red Dwarf (thanks to Lister falling asleep at the wheel), and find themselves in "Vampire GELF" territory. The Cat becomes terrified when he hears that Vampire GELFs feed on the blood of virgins, though hastily tries to deny this, claiming he has "two lady cats" on Z Deck.
While correcting their course, they spot a ship about to fall into a black hole. Learning that the ship is populated by a "Mercenoid" and an organic prisoner, the crew mount a rescue. After a brief confrontation, the Mercenoid is killed, and the prisoner is located. The prisoner is revealed to be a female Felis Sapien, to the Cat's delight, and is brought back to Red Dwarf, where the cat plans a date with her, culminating in him finally having sex.
Soon after, Kryten discovers that the "lady cat" is actually a Polymorph, disguised as a Felis Sapien. By the time the rest of the crew alert the Cat of this, he has already been impregnated with the Polymorph's offspring. All attempts to remove the Morphling eggs before they hatch prove futile, leaving the crew no choice but to allow them be born. Afterwards, they try to dispose of them, but the Cat, having been affected by a pheromone the Morphlings produce, runs off with them instead.
In an attempt to take out the Morphlings before they are fully grown, Krytens uses a "personality tuck" machine they had salvaged from a medical frigate earlier to remove Lister's emotions, making him practically invisible to the Morphlings. Lister succeeds in killing two of them, but the others pose as the rest of the crew members, sowing confusion and paranoia until the Cat shows up and kills all the remaining Morphlings.
That night, Lister comes to talk to the Cat, telling him that being a virgin isn't such a bad thing (hastily adding that he doesn't think he is). After Lister leaves, two lady cats pop up in bed with the Cat. One of them points out that the Cat is just dreaming this, but the Cat doesn't care.
"Can of Worms" contains examples of:
- Bounty Hunter: The "Mercenoids" fill this quota.
- Continuity Nod:
- The Cat's old "That's mine, this is mine" routine from season 1 makes a return.
- The crew once again encounter a Polymorph.
- Emotion Eater: The Polymorphs, as usual.
- Emotion Suppression: Kryten uses the "personality tuck" machine to remove Lister's emotions, making him practically invisible to the Morphlings.
- Gone Horribly Wrong: Removing Lister's emotions turns him into an unempathetic killer who doesn't bat an eye at the thought of accidentally killing the Cat.
- "How Did You Know?" "I Didn't.": When the emotionless Lister kills a Polymorph posing as the Cat, he provides this rationale.
- Inverted Trope when Cat ends the episode gunning down all the Dwarfers' doubles in the end. "A momma always knows."
- Mr. Seahorse: The Cat is impregnated with Morphling eggs.
- Never Trust a Trailer: The publicity for this episode avoided mentioning the return of the Polymorph, and suggested it was going to be a story about the Cat falling in love with another of his own kind.
- Nice Job Breaking It, Hero!: The Mercenoid was trying to dispose of the Polymorph by flying his ship into the black hole, but the Dwarfers inadvertantly ruined his plan.
- Of Course I'm Not a Virgin: The Cat tries to invoke this. The others don't believe him, but humor him anyway.
- Stylistic Suck: Despite the series previously having used much more convincing (well, for the time) morphing effects for the Polymorph's cousin, the Emohawk back in Series VI, the Morphlings' changing forms is depicted using the same jump-cut technique used back in the first appearance of the species all the way back in Series III.