The Doctor and Peri face enemies at every turn on the planet Magnus. There's the Time Lord bully Anzor, who made the Doctor's life hell during his time at the Academy. There's also Rana Zandusia, the matriarchal ruler of the planet, who seeks to prise the secret of time travel from these alien visitors.
 
Also on Magnus is the slug-like Sil, still bitter from his defeat on the planet Varos and seeking to make his fortune from the most potentially destructive ends. And, deep within the planet, there is something else. Another old enemy of the Doctor's. And the future is looking decidedly colder...