Parts: 4
Writer: Anthony Steven (with Eric Saward)
Airdates: 22 March 1984 - 30 March 1984
Production Code: 6S
Creature Feature: Gastropods
Story: Peri finds herself very unsure of where she stands with the Doctor as his recent regeneration seems to have turned him slightly unstable. After suffering a mild fit in the wardrobe room, then choosing a new outfit that she considers to be absolutely hideous, the last straw comes when he turns on her, proclaiming her to be first an evil faerie, then an alien spy, and attempts to strangle her. The Doctor remembers little of his fits, but Peri cannot forget them. Deciding that he needs to atone for his sins, the Doctor declares that he will become a hermit on the desolate planet Titan Three, with Peri to care for his few needs.
Meanwhile, a pair of twins, both geniuses in mathematics, have been kidnapped from their home by an elderly man in the employ of Mestor, a large slug-like creature. The twins are taken to Titan Three in a stolen ship, which is followed by Lieutenant Hugo Lang, an Interplanetary Pursuit Officer. Lang's ship crashes on the planet, where he is rescued by the Doctor and Peri.
The prospect of a mystery causes all thoughts of hermitage to flee from the Doctor's mind. But soon, he, Peri and Hugo find themselves caught up in Mestor's horrible scheme to spread his kind all across the galaxy. If the Doctor cannot stop him, every planet touched by Mestor's legacy will be utterly devoured.