Plot

The Red Dwarf crew encounter the Polymorph, a genetically engineered space mutant that drains the emotions from humans, that has arrived on-board Red Dwarf.

An escape pod containing a scientifically-created genetic life-form approaches Red Dwarf and the creature inside escapes and infiltrates the ship. Once aboard it assumes the shape of any object it desires to approach it's prey, settling on a basketball. Lister is preparing dinner in the quarters when Kryten comes in to clean with a vacuum hose attached to his groinal socket, he goes to change the dustbag when Lister's dinner invitee Cat enters.

Cat is a little put-off by Lister's usage of medical equipment as kitchen utensils, Lister insists all the equipment is clean. The food is served and before Cat can eat Lister offers lemon juice out of a syringe, Cat leaves in disgust, Lister eats by himself when the Polymorph in disguise as the basketball enters, Lister puts the ball down on the table to check where it came from and the Polymorph assumes the form of a sausage on Lister's plate.

Rimmer is watching home movies when Kryten comes in, Kryten accidentally insults Rimmer's mother and leaves in embarrassment. Holly interrupts the viewing to warn Rimmer of an alien life-form on-board. Lister is enjoying his meal when Kryten walks in, Lister goes to stab his food with a scalpel when the Polymorph attacks him, it scoots under the cooker and disappears.

Lister puts on a pair of boxer shorts that were laying on the floor and they start to contract, Lister falls to the floor and Kryten attempts to remove them with his vacuum, Rimmer walks in and sees the two on the floor in a precarious position, Kryten has removed the boxers and leaves them on the top bunk bed.

The boxers have vanished and in their place Kryten picks up a boa constrictor and throws it at Lister in panic. Lister hurriedly stuffs it into the waste bin. Lister gets his scalpel and opens the bin, the Polymorph's true form emerges, an eight-foot tall Xenomorph expy, the creature shoots out its tongue which attaches to Lister's forehead and Lister faints.

While Lister is recovering in the medi-lab Holly and Kryten reveal their findings on the creature, it was created as the perfect assassin to change shape and approach targets undetected. However the creature was insane, it scans the minds of a target finding a weakness, provokes a negative emotion to its limit and drains the emotion from the victim's mind. In Lister's case, stealing his fear. A now fearless Lister wakes up and wants immediate revenge, Kryten has to sedate him back into unconsciousness.

The crew travel into the ship's cargo bay armed with two-handed firearms called Bazookoids and hunt the creature, Rimmer calls a false alarm and heat-seekers are fired into the distance which return and pursue Cat as he is the only one generating heat. Cat outruns the heat-seekers trapping them in an airlock, Rimmer and Kryten attempt to locate Cat when he's approached by a mysterious woman who flatters him, with his vanity swollen she introduces herself as the genetic mutant and drains Cat of his vanity.

Kryten finds Cat downed and discovers his vanity drained, Rimmer suspiciously blames Kryten for the Cat provoking Kryten's guilt, the Polymorph claims another victim and flees when the real Rimmer appears.

They return to the medi-lab and find Rimmer's mother with Lister, she claims to have had sex with Lister driving Rimmer's anger to breaking point, now all Dwarfers are drained of a negative emotion.

Sometime later the Dwarfers are discussing battle plans, but without the emotions that make them effective they can't work together, eventually deciding to head to the cargo bay. They reach the airlock with the heat-seekers inside unsealing the door, the heat-seekers zoom at them and they duck in time for the Polymorph behind them to be slain and their emotions returned.

A brief shot of the escape pod shows a second Polymorph was inside and on the ship having taken the form of a second Lister.