In 2084, two opposing superpower blocs struggle for dominance. The underwater Sea Base Four is armed with nuclear weapons aimed at the opposing bloc. The weapons are controlled by the human mind. The crew is led by commander Vorshak (Tom Adams) and the weapons controlled by Lieutenant Michaels, who is killed before the start of the story and replaced by the hapless Ensign Maddox (Martin Neil). On the bridge of Sea Base Four, Vorshak and Bulik (Nigel Humphrys) notice a glitch on their sensors, which is a Silurian battlecruiser commanded by Icthar (Norman Corner), sole survival of the Silurian Triad once confronted by the Third Doctor.
As the TARDIS materializes in space, it is attacked by Sentinel Six, a robotic weapons system. To save the TARDIS, the Doctor lands in Sea Base Four, now beginning a practice missile run. The inexperienced Maddox faints and Commander Vorshak is looking for a replacement. Nilson (Ian McCulloch) and Doctor Solow (Ingrid Pitt) are enemy agents and plan to program Maddox to destroy the system, which they attend to in the Psycho-Surgery Unit.
The Doctor’s presence is detected. He overloads the Base Reactor to avoid capture, but the TARDIS crew are taken prisoner. Meanwhile the Silurians revive the Sea Devil Warriors of Elite Group One (which the Third Doctor also once confronted) and their Commander Sauvix (Christopher Farries). Both the Silurians and the Sea Devils attack the base, and the Doctor tries to warn Vorshak not to fire on them, but he does, and the Base’s defences are neutralised by the Silurian’s Deflection Beam. The Silurians send in the Myrka, a huge sea-monster, to attack Airlock One, while the Sea Devils attack Airlock Five.
During the attacks, Maddox tampers with the equipment, and when Ensign Karina (Nitza Saul) becomes suspicious of him, Nilson forces Maddox to kill her. The Myrka penetrates the base, trapping the Doctor and Tegan until Turlough gets the airlock door open. The Myrka heads for the bridge, killing people as it goes until the Doctor destroys it with an ultra-violet light generator.
The Silurians prepare something called a Manipulator. The Sea Devils break through Airlock Five and kill their way to the bridge. Nilson is revealed as a traitor when he takes Tegan hostage to escape. The Doctor blinds him with UV light and the Sea Devils kill him. The Doctor and Tegan are taken prisoner by the Silurians. The Doctor recognizes Icthar and confronts him. Icthar reveals his plan to get mankind to destroy itself by triggering Global War. The Manipulator is connected to the system.
The Doctor escapes from the bridge and commandeers several tanks of hexachromite gas, which is lethal to reptiles. A Sea Devil tries to shoot him. He misses and hits one of the cylinders, spraying himself with the gas, and he dissolves. Preston (Tara Ward) urges the Doctor to spray all of them. He refuses at first, calling it genocide, but at Turlough’s urging, he connects the cylinders to an air pump. Sauvix catches him at it. Preston grabs a gun but is killed by Sauvix, who is in turn killed by Bulic. As the Silurians prepare to fire the missiles, the Doctor feeds the gas into the ventilation system.
The Warriors begin to collapse, but the Doctor tells Tegan and Turlough to give them oxygen. He succeeds in stopping the missile launch, but Vorshak is killed by Icthar, who is killed by Turlough. The Doctor is in despair at the wiping out of species that predate Man on Earth.
Production was interrupted by Margaret Thatcher’s election call, the BBC pretty much abandoned Doctor Who for election coverage, and many scenes were filmed without benefit of rehearsal. It shows. The Myrka costume was completed assembled an hour before it was used. The puppeteers inside were nearly asphyxiated by glue and it was covered in wet paint. It was so awkward that the puppeteers, with no rehearsal, could barely make it creep forward. Its victims had to throw themselves into its embrace to be killed, like a scene from an Ed Wood film. Competition for the most pathetic monster on Doctor Who is fierce—some from the First Doctor’s tenure are favorites—but I think the Myrka takes the prize fins down.
The Silurian and Sea Devil costumes were better, but not much, with visible zippers, glimpses of the actors inside, and parts of the costume falling off. During the filming, both Peter Davison and Janet Fielding announced they were leaving. The original idea, to bring back two ancient sentient reptilian Earth species to attack the humans who seized their planet, was interesting, but hardly anything worked out. Perhaps symbolic of the whole exercise: the left-hand airlock door clearly says, “Open Other Door First,” and the right-hand door says exactly the same thing.