The Fourth Doctor's first trip in the TARDIS brings him, with Sarah Jane and a very skeptical Harry Sullivan, many thousand years intot he future to Nerva, a space station in Earth's orbit. The Doctor is amazed to discover that Nerva is nothing less than an ark for the human race. Having evacuated Earth when solar activity threatened all life on the planet, these few hundred survivors have been drifting in suspended animation for thousands of years. Even more perplexing is the discovery that someone has sabotaged Nerva's vital systems. The Doctor reactivates the systems, and one by one, the human race begins to awake from its enforced deep sleep. The future of humankind appears secure until the Doctor learns that the larvae of the Wirrn, a hostile insect race, are also aboard.