Turlough has changed his mind about going home, and the Doctor plans to show Tegan something of Earth’s future. The TARDIS materialises in space where it is intercepted and attacked by Sentinel Six (voice: unknown), a robot weapons system. The Doctor manages to avoid the ship’s destruction be performing a materialisation flip-flop. 

The TARDIS then materialises in an underwater base, Sea Base 4, around the year 2084. The base is being monitored from a nearby battle cruiser by a group of three Silurians – Icthar (Norman Comer), sole survivor of the Silurian Triad; Tarpok (Vincent Brimble); and Scibus (Stuart Blake) – who later revive the Sea Devil warriors of Elite Group One (Steve Kelly, Chris Wolfe, Jules Walters, Mike Braben, Dave Ould), led by Sauvix (Christopher Farries). 

The Sea Base is commanded by Vorshak (Tom Adams), with Bulic (Nigel Humphries) as his second in command. There is a cold war in progress, and the Base goes through frequent practice missile runs for which a sync operator in required to link with the computer and thereby control the firing of the missiles. Maddox (Martin Neil), the temporary sync operator, is uncertain of his skill at the job. 

In charge of the Base’s medical lab is Doctor Solow (Ingrid Pitt) who is in truth an enemy agent in league with Nilson (Ian McCulloch). They plan to program Maddox to destroy the computer circuitry. To do this they ask Vorshak to release Maddox’s duplicate program disk under the pretext of helping the sync operator cope with his job. Vorshak does so, and Maddox is programmed in the Base’s psycho-surgery unit. 

The Doctor’s presence on the Sea Base is detected when Turlough summons an elevator. The Time Lord programs the base’s reactor to overload in an attempt to avoid capture. This fails, however, and the time travellers are all taken prisoner. The Base’s security officer, Preston (Tara Ward), also finds the TARDIS. 

The Silurians launch an attack on the base and the Doctor, recognising their ship on the monitor screen, tries to prevent Vorshak from firing on them. Vorshak ignores him, however, and the Base’s defences are neutralised as the Silurians deflect its fire. The Silurians then unleash a huge reptilian creature called the Myrka (William Perrie, John Asquith). While this is attacking Airlock 1, the Sea Devils launch an assault on Airlock 5. 

Using the attacks as a diversion, Solow and Nilson activate Maddox, who starts tampering with the equipment. Karina (Nitza Saul), another of the Base personnel, is suspicious, and Nilson makes Maddox kill her. 

The Myrka forces its way into the base, temporarily trapping the Doctor and Tegan until Turlough gets the inner airlock door opened to save them. The creature starts making its way towards the bridge, killing people by electrocution. Meanwhile the Silurians make ready a device called the manipulator and prepare to enter the base themselves. The Sea Devils break through Airlock 5 and start the push for the bridge, killing any crew that stand in their way. 

The Doctor attacks the Myrka using an ultra-violet light generator which blinds and then kills it. However, this comes too late to save Doctor Solow, who was foolish enough to try to engage the creature in kung fu on her way to an escape pod. 

Nilson, who has been unmasked as a traitor by Vorshak, takes Tegan hostage. The Doctor blinds him with the ultra-violet device, and a group of Sea Devils appear and kill him. The Doctor and Tegan are taken as prisoners to the bridge, which is now under the control of the Silurians. The Doctor knows Icthar of old and thought he had been killed. The Silurians’ plan is now revealed: they intend to get mankind to destroy itself by triggering a global war. They fix Maddox’s tampering and connect the manipulator to the systems. 

The Doctor escapes from the bridge and tries to find something to use against the reptiles. He discovers some cylinders of hexachromite gas, which is lethal to them. Sauvix kills Preston before being sprayed with gas and killed by Bulic. As the Silurians prepare to fire the missiles, the Doctor bows to pressure from Bulic, Tegan and Turlough (who have also got free) and agrees that they should feed the gas into the ventilation system. Bulic stays in the chemical store to ensure that the gas keeps flowing, while the Doctor and his companions leave for the bridge to try to stop the Silurians.

The reptiles collapse under the effects of the gas and the Doctor tells Tegan and Turlough to give the Silurians oxygen to keep them alive. Meanwhile, aided by Vorshak, he tries to stop the missiles by linking himself into the equipment as sync operator. The Doctor succeeds, but Vorshak is killed by one of the Silurians before the Silurian itself is killed by Turlough. The Doctor and his companions have won through, but everyone bar themselves and Bulic is dead. The Doctor is left to muse that there should have been another way.

_{Synopsis from Doctor Who: The Fifth Doctor Handbook by David J. Howe, Mark Stammers and Stephen James Walker, reprinted with permission; further reproduction is not permitted. Available from Telos}