"Polymorph" is the third episode of science fiction sitcom Red Dwarf Series III, and the fifteenth in the series run. It premiered on the British television channel BBC2 on 28 November 1989. It is considered by some to be the series' best. Written by Rob Grant and Doug Naylor, and directed by Ed Bye, the episode has the crew fighting a shapeshifting, emotion-stealing creature. It is the only Red Dwarf episode to feature a pre-credits warning about the content. The episode was re-mastered, along with the rest of the first three series, in 1998.

A non-human life-form with shape-changing properties escapes from a waste pod floating in space and gets aboard Red Dwarf. Although Holly (Hattie Hayridge) detects it, despite scepticism by Arnold Rimmer (Chris Barrie) that it's a false alarm, the creature attacks Dave Lister (Craig Charles) while he is having a meal. As Rimmer and Cat (Danny John-Jules) wait for him to recover in the Medical Unit, Kryten (Robert Llewellyn) and Holly reveal the creature to be a man-made genetic mutant called a "Polymorph", designed to leech off its target's negative emotions, revealing that when Lister suffered immense fear when the creature attacked, it drained him of it and has now left him all too eager to face any danger.

To avoid it, the others seal Lister away and prepare to escape with a Starbug for a getaway. However, the Polymorph manages to capture each on their own, after an accident with heat-seeking bazookoids, and trick them into exposing a negative emotion – Cat loses his vanity; Kryten loses his guilt; and Rimmer loses his anger. All four find themselves at a disadvantage against the creature as a result, but opt to confront it down in the ship's cargo decks. Although things don't start well, they defeat it by pure luck thanks to their earlier accident, causing them to recover their lost emotions. As the group leave the cargo decks, they are unaware of a second Polymorph having boarded the ship from the same waste pod floating past Red Dwarf, which follows them, mimicking Lister's appearance.

When the episode was remastered, Doug Naylor opted to change the ending of the story – in the remastered version, the second Polymorph makes its appearance as a bouncing ball, whereupon a caption explains that it was less intelligent than the first, stowed away in Lister's clean underpants drawer, and died of old age many years later. Naylor stated the change of ending was due to his dislike of fans asking him about the second creature's fate in the original ending.^{[citation needed]}

Director Ed Bye drew inspiration for the look and design of the episode from watching the 1979 film Alien just before shooting began. For the cargo bay scenes a warehouse in Manchester was hired for the shooting. Empty cardboard boxes were strategically stacked up and down the warehouse floor to give the illusion of a stocked cargo bay.

Although the episode had a pre-credits warning, about the episode's content, this was more of a plot device as the episode was broadcast past the 9:00 pm watershed. During the filming of the Polymorph morphing into Lister's boxer shorts scene the audience laughed so loud for so long that Chris Barrie had to wait several minutes until things had calmed down (the scene is now considered by many to be one of the funniest of the entire series). This also provided extra work in the editing room as the sequence required more work than usual to edit together a workable shot.

The episode also had some of the show's most effects intensive scenes done yet. Animatronic versions of both the small and large Polymorph creature versions were created. The small version was voiced by production manager Mike Agnew. The larger model, which was prone to tipping over, collapsed before the final shot. Blue screen was used to shoot the creature and added to existing footage of the crew. To achieve the Polymorph morphing into all the different objects the traditional locked-off camera and jump-cut procedure was used. The newly introduced Bazookoids provided more video effects as two heat-seeking laser bolts were fired and ended up chasing the Cat. The heat-seekers were given sound effects borrowed from the lightsabers of Star Wars.

Guest appearances included actress Frances Barber who took on a scene as 'Genny Mutant', Kalli Greenwood, who played Rimmer's mother, and the show's first appearance of Rimmer as a young boy, played by Simon Gaffney.