The story takes place on a space ship to which the TARDIS has accidentally attached itself. The space ship offers to treat those afflicted with lazar disease. The serial endfs with the doctor's companion Nyssa choosing to stay on Terminus and treat the lepers.
The story was written by Steve Gallagher and directed by Mary Ridge. The cast also includes R. J. Bell (The Garm), Peter Benson (Bor), Andrew Burt (Valgard), Valentine Dyall (Black Guardian), Liza Goddard (Kari), Dominic Guard (Olvir), Tim Munro (Sigurd), Martin Potter (Eirak) and Rachel Weaver (Inga).
See also List of Doctor Who serials