The Doctor's time-travelling 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 materialises 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 has 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...