The year is 2084, and there is a cold war in progress. The world is divided into two opposing super power blocs. One of the blocs have created a secret underwater base, Sea Base 4, which is strategically positioned and has nuclear weapons aimed at the opposing bloc. As a security measure, the Seabase nuclear weapons can not be activated, unless a trained human operator can "sync" his/her mind with the computer and authorize their deployment.
The base's crew is led by Commander Vorshak and his senior officers, Nilson, Bulic, Security Chief Preston, and Lt. Michaels, the base's sync operator. Lt. Michaels is mysteriously killed before the start of the story, and as a result, his inexperienced apprentice, Ensign Maddox, is forced to assume Michaels' official responsibilities. The story begins on the bridge of Sea Base 4. Vorshak and Bulic noticed something strange on their long range sensors, but dismiss it as being a trivial glitch. In reality, the glitch is a Silurian battleship led by Icthar, the sole survivor of the Silurian Triad and his subordinates, Tarpok and Scibus, who are monitoring Sea Base 4.
Inside the TARDIS, Turlough has changed his mind about going home, and the Doctor plans to show Tegan something of Earth’s future. As the TARDIS materialises in space, it is attacked by Sentinel Six, a robot weapons system. In a blink of an eye, the Doctor manages to save the TARDIS by materializing on to Sea Base 4.
Sea Base 4 under goes a practice missile run, but Maddox, the temporary sync operator, is uncertain of his skill at the job. When Maddox faints after the practice run, Vorshak begins to realize that the function of the base will continue to be compromised until either Maddox lives up to his duties or a replacement is assigned. Nilson and the Base’s chief medical officer, Doctor Solow (played by Ingrid Pitt),who are enemy agents for the opposing bloc, 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 a lift. 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. Preston also finds the TARDIS.
The Silurians revive the Sea Devil warriors of Elite Group One and their brilliant commander, Sauvix. The Silurians and Sea Devils launch an attack on the base and the Doctor, recognising their ship on the monitor screen, tries to warn Vorshak not to fire on them. Vorshak ignores him, and, as a result, the Base’s defences are neutralised by the Silurians' deflection beam. The Silurians then dispatch the Myrka, a large marine monster, who attacks Airlock 1, and the Sea Devils, who assault Airlock 5 of the sea base.
During the attacks, Solow and Nilson activate Maddox, who tampers with the equipment. When Ensign Karina becomes suspicious 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. Doctor Solow become one of the Myrka's victim when she foolishly tries to engage the creature in combat while she was on her way to an escape pod. The Doctor eventually destroys the Myrka by using an ultra-violet light generator.
The Silurians prime up a device called the manipulator and prepare to arrive on the base. The Sea Devils break through Airlock 5 and start the push for the bridge, killing any crew that stand in their way. Solow's accomplice, Nilson, is revealed as a traitor and he attempts to escape by taking 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 recognizes Icthar and confronts him about the massacre of the crew of Sea Base 4. Icthar reveals his group intends to get mankind to destroy itself by triggering a global war. They undo the damage caused by Maddox’s sabotage 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 all reptile life. A Sea Devil discovers the Doctor's presence and attempts to shoot him. He misses the Doctor and hits one of the gas containers which sprays all over the warrior. As a result, the warrior begins to dissolve. Preston urges the Doctor to use the gas on all of the Silurians and Sea Devils. The Doctor adamantly refuses and accuses Preston of advocating genocide. The Doctor changes his mind when Turlough reminds him of what the Silurians intend to do if they launch the missiles, and when he is unable to find anything else harmful but less lethal - so begins to connect the gas containers to a central air pump. The Doctor is discovered by Sauvix before he can turn the pump on. Preston grabs a gun, but is killed by Sauvix before he is sprayed with gas and killed by Bulic. As the Silurians prepare to fire the missiles, the Doctor feeds 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 warriors begin to collapse from the gas and the Doctor tells Tegan and Turlough to give the Silurians oxygen to keep them alive. The Doctor, who is aided by Vorshak, tries to stop the missiles by linking himself into the equipment as the sync operator. The Doctor succeeds, but Vorshak is killed by Icthar. Then Itchar, himself, is killed by Turlough. Then it is all over. The Doctor, his companions, and Bulic are the only survivors. The Doctor is left in despair and he simply says, "There should have been another way."
Doctor Peter Davison (Fifth Doctor)
Companions Janet Fielding (Tegan Jovanka)
Mark Strickson (Vislor Turlough)
Tom Adams — Vorshak
Ian McCulloch — Nilson
Ingrid Pitt — Solow
Nigel Humphreys — Bulic
Tara Ward — Preston
Martin Neil — Maddox
Nitza Saul — Karina
James Coombes — Paroli
Norman Comer — Ichtar
Stuart Blake — Scibus
Vincent Brimble — Tarpok
Christopher Farries — Sauvix
Writer Johnny Byrne
Director Pennant Roberts
Script editor Eric Saward
Producer John Nathan-Turner
Executive producer(s) None
Production code 6L
Series Season 21
Length 4 episodes, 25 minutes each
Originally broadcast January 5–January 13, 1984