Parts: 4
Writer: Johnny Byrne
Airdates: 5 January 1984 - 13 January 1984
Production Code: 6L

Doctor: fifth
Companions: Tegan, Turlough

Creature Feature: Silurians, Sea Devils, Myrka

Story: To Turlough's annoyance, the Doctor has promised Tegan a visit to Earth's future, but he overshoots and the TARDIS materializes in the planet's atmosphere around 2084. Progress has not changed the Earth much - there are still warring factions on the surface and excessive measures are still being taken to ensure victory for either side. When the orbiting TARDIS is confronted and attacked by a computerized sentinel, the Doctor must force a desperate materialization.

When the TARDIS re-materializes, it is deep underwater aboard Sea Base Four. The base staff, alarmed by the mysterious disappearance of the unidentified object in Earth's orbit as well as by the mysterious disappearance of one of their own probes, are already on edge, and when the TARDIS crew accidentally reveal their presence, an immediate offensive is taken.

When the Doctor is discovered tampering with the reactor, in an attempt to provide a distraction that will allow the TARDIS crew to escape unnoticed, he is immediately suspected to be the cause of all their problems. But what the Base crew do not realize is that there is a real saboteur in their midst...and as if that were not enough to worry about, a craft is slowly approaching. Inside, it carries some of the Doctor's old enemies...along with a new surprise. With the help of Tegan and Turlough, the Doctor must gain the trust of the Base crew, uncover the saboteur, and stop the Base and its missiles from falling into the clutches of creatures who want to turn mankind against itself, or the planet will be wiped out by man's own destructive tendencies.