The latest extra-terrestials to be rescued by the Moon Alpha Base crew are Pasc and his young son Etrec. They are from the usually peaceful planet of Archaron and they claim that they are victims of a rebellion, engineered by Pasc's wife Lyra. However, Helena discovers that they have both been infected with a virus which makes them considerably less friendly and more deadly than their initial claim.

The Alphans land on a planet seemingly devoid of any human existence but being run by a group of machines. The machines explain to the visitors that their human overlords are in a state of regeneration but the chief machine, the Guardian,sees the Alphans as posing a threat and it is down to two newly regenerated females known as A and B to decide their fates.

Several crew members explore the planet Sunin, including botanist Shermeen Williams, who appears to have visionary gifts. She links up with an alien, called Vindrus, who wants her help in transferring his body and that of his race, into solid matter, giving them proper human form so that they can join the universe as real matter. However, for every one of his race made flesh, an Alphan must lose their matter.

A spaceship arrives on Moonbase Alpha carrying a prisoner, whom Maya is astonished to learn is Dorzak, a survivor from her planet, Psychon. But can he, a man of peace, really be guilty of the crimes of violence for which he is being exiled?