The Moon is pulled into a
spacetime anomaly which duplicates it, as well as Moonbase Alpha and its crew. After the Moon re-enters normal space, its duplicate speeds away and the Alphans' doubles vanish. It is discovered that the Moon is in a new region of space and approaching a
planetary system. Regina Kesslann emerges from the ordeal no longer recognising her life on Alpha. Although she has never been married, she claims that her "husband" – Alan Carter – is dead, as is Commander Koenig. Believing that the Moon has travelled
faster than light, Professor Bergman ponders whether time could be affecting Kesslann's mind: she may be experiencing a past or future life. Scans reveal that the planetary system is Earth's own Solar System and the Moon is re-entering Earth orbit. As the Alphans prepare to evacuate, believing that they are home, Kesslann has a vision of herself with an older and sadder face, which then turns into a skull. Her head wracked by pain, she dies in Carter's arms. Dr Russell's autopsy finds that Kesslann's brain was duplicated inside her head, causing fatal
intracranial pressure. Bergman discovers that a shift in
Earth's tilt has caused global environmental collapse. The only remaining habitable area is
Santa Maria, California. The duplicated Moon reappears, emitting Alpha's signal. Flying there an Eagle, Koenig and Carter discover a duplicated Alpha devoid of crew and equipment. Investigating a crashed Eagle, they find two dead astronauts – themselves. The duplicated Moon is accelerating to collide with the original. Although evacuation to Santa Maria is now vital, Russell insists on limiting the first landing party to Koenig, Carter, and herself as medical officer. If any Alphans meet their doubles, they risk suffering Kesslann's fate, but Koenig and Carter are safe because their counterparts are dead. Koenig, Russell and Carter touch down in Santa Maria and find a settlement inhabited by the ageing doubles of Russell, Bergman and others. Duplicate Kesslann has just died. Duplicate Russell states that she is about to die, but welcomes it because she will be reunited with her late husband – duplicate Koenig. She kisses Koenig and dies in his arms. The settlers demand that the new arrivals leave. Bergman, unsure whether this is a past or future Earth, deduces that the two Alpha crews are the same people trapped in different times, and considering what happened to Kesslann and Russell it would be disastrous for any more Alphans to come to Earth. He predicts that the two Moons' collision will put things right, leaving one Moon and one set of Alphans in one time. The new arrivals must be on their Moon when the correction occurs. Koenig, Russell and Carter return home and the Alphans await the collision. The Moons touch, creating a schism that merges them together and transports the one Moon to yet another part of the cosmos. The Alphans wonder whether their doubles survived or even existed at all. However, Russell still has a bouquet of flowers that Bergman's double picked for her. ==Regular cast==