Medusa, torn from orbit around Proxima Centauri by a comet, was sent careering through space. Ages later, with Medusa now in orbit between Uranus and Neptune, Adam and Shem escape the autocratic all-female Medusan society and head to earth.
Surviving the crash, Adam and Shem steal a police car and go on the run. Supreme Councillors Octavia - head of security - and Fulvia - Adam's mistress - follow them to earth to retrieve them. Many misunderstandings with the humans ensue.
Adam and Shem shut themselves inside an ancient defensive manor and take the curator hostage. Octavia activates the nightmare cannon, bringing their fears to life to drive them into the open, where they are taken into custody by the police.
While Shem repairs her ship, Fulvia allows herself to be used as a test subject for a human thought-transference machine similar to the ones on Medusa - but not similar enough, as it causes pain and distress. Who will save her?
Adam and Shem hatch a plot that should convince Fulvia to release the human hostages while still allowing the refugee Medusans to remain on earth. It involves Fulvia and Adam setting up home as a couple. What could possibly go wrong?
Prof Evans visits Medusa to discuss the return of the hostages with the President - who they find apparently dead. Liz is accused of causing her death and sentenced to die on the surface. Can the professor save her?
On the run, Adam and Shem find shelter in an apartment, but have been seen and reported. Adam is arrested, but Shem is rescued by Rose, whose fiancé has left her. The two start to grow close - but the police are closing in.
At Octavia's request, Liz - with Rudi - investigates the abandoned records section. Rudi's searching for a way off Medusa: what Liz finds is far more sinister and distressing, an ancient experiment to create artificial life.
Agreement now reached on the exchange of hostages, the two ships head off from earth and Medusa. But a deadly ancient enemy of the Medusans has found them. The ships have no weapons: can Rudi come up with a plan to save them all?