The TARDIS's first port of call is a seemingly deserted space-wheel orbiting Earth in the far future around the 131st century. On board, the Doctor, Sarah and Harry discover that the wheel is an Ark containing the last surviving remnants of the Human race in suspended animation. The Earth was evacuated thousands of years before as solar flare activity threatened to destroy all life, and the cream of humanity was selected to board Space Station Nerva, now renamed the Ark.

The Doctor arrives soon after a visit by a single Wirrn Queen. The Wirrn are an insect species first encountered by humankind on Andromeda. They lay their eggs close to a food source so that their larvae have available nourishment, both physical and mental. The Wirrn larvae also secrete a slime which can physically invade and transform other creatures into Wirrn, retaining the knowledge and intelligence of the host. 

On the Ark, the Wirrn Queen had already taken the body of Technician Dune, and with his knowledge of the ship, had laid her eggs close to the solar stacks - the Ark's power source - intending to use the remaining sleeping humans as food stock for her young. This task completed, the Queen had hidden herself away in a storage locker and died. 

The Doctor reactivates the controls of the Ark, and accidentally revives a small complement of the crew. The leader, Lasar (nicknamed Noah), becomes infected by a Wirrn grub when he checks the solar stacks and is slowly taken over. Vira, Libri, Lycett and Rogin, however, gradually grow to trust the Doctor and help in his plan to lure the hatched Wirrn insects into a transport ship before ejecting it into space. 

With one final act of human sacrifice the Noah/Wirrn neglects to set the rocket stabilisers, resulting in the shuttle exploding in space and destroying the invading Wirrn. 

The Doctor offers to pop down to Earth using the Ark's transmat system to ensure that the diode receptor beacons are correctly tuned. Sarah and Harry elect to accompany him and the three leave.

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