Mawdryn Undead — Peter Grimwade

The Doctor's time-traveling machine is trapped in the flight-path of an alien spacecraft in orbit around the Earth. To avoid the fatal impact of a head-on collision the TARDIS resorts to the only escape possible and materializes on board the on-coming liner.

This solves the immediate problem, but a new difficulty arises--the TARDIS cannot get off the ship until a radio signal transmitting from Earth had been disconnected.

The Doctor sets off in a Transmat Capsule, having programmed the TARDIS to enable Tegan and Nyssa to follow him once he has dealt with the interference.

Naturally enough, things don't go quite as planned...