The year is 2084, and Earth is divided into two hostile blocs which eye each other suspiciously for the slightest wrong move, armed with proton missiles which could destroy all life on Earth while leaving the planet itself intact. But there are other forms of life on Earth… A Silurian battle cruiser lands near a Sea-Devil hibernation chamber near one of the human blocs’ Sea Bases, and when an inquisitive probe is launched the Silurians release a “Myrka” to destroy it. The crew of the Base aren’t sure how to react to the probe’s destruction, as their sensors are picking up nothing but marine life. The base’s temporary synch operator, Maddox, whose job it is to link to the computer in times of crisis, is unprepared for the emergency; he’s only a student thrust into this position by the accidental death of the base’s former synch op, Michels. Maddox’s friend Karina talks him back to work, unaware that his suspicions about Michels’ death are justified; the base’s second-in-command and doctor, Nilson and Solow, are enemy agents who murdered Michels in order to put the unstable Maddox in a position where they can use him.
Turlough, for reasons of his own, has decided to remain aboard the TARDIS. The Doctor materializes in orbit around Earth in order to show Tegan some of her planet’s future, but they are shot down by a satellite defense system and the Doctor is forced to make an emergency materialization on the Sea Base. Their encounter with the satellite having coincided with the destruction of the Base’s marine probe, the Base’s computer triggers what could be a missile run. Maddox is forced to synch up to the computer in preparation for what could be the first strike of the last war. The run turns out to be a simulation to test the crew’s battle readiness, but the stress causes Maddox to pass out, and he is taken to the psychosurgical unit for care. With Maddox incapacitated; the Base is defenseless.
The Silurians, meanwhile, have docked with the hibernation chambers near the Sea Base, and are preparing to revive the Sea Devil patrol there. As the Silurians Scibus and Tarpok activate the revival circuits, Ichtar — the sole survivor of the Silurian Triad — waits patiently. The process will take some time, and the long period of hibernation may have caused the Sea Devils’ muscles to atrophy; all they can do is wait.