Parts: 4
Writer: Steve Gallagher
Airdates: 3 January 1981 - 24 January 1981
Production Code: 5S

Doctor: fourth
Companions: the second Romana, Adric, K-9 mark two

Creature Feature: Tharils

Story: During flight, the TARDIS is entered by a leonine creature who appears to move out of phase with the ship's crew. After adjusting several of the settings, he introduces himself as Biroc, then leaves. The Doctor discovers that the TARDIS has been set to zero co-ordinates - the interface between N-Space's positive co-ordinates and the negative co-ordinates of E-Space - and that K-9 has been badly damaged by the Time Winds which entered the console room when the doors were opened.

Having landed in the interface, the Doctor follows Biroc to a Gateway. Romana is captured by the crew of another trapped vessel. Because Biroc, the time-sensitive Tharil they use for navigation, has escaped, they use Romana to accomplish the same task - to get them to the Gateway. Once there, they confront the Doctor, but he is able to slip through a mirror that leads to the Tharils' world.

The crew of the other vessels are slavers, trading in time-sensitives. But the only material that will contain time-sensitives is dwarf-star alloy, and the hull of the slavers' vessel is causing the interface to collapse. Unless the Doctor can stop it, they may be stranded in E-Space forever.