The crew of Moonbase Alpha pause work to celebrate the arrival of the first child born on the Moon
since it broke away from Earth. It is the son of Sue Crawford and her husband Jack, a technician in one of Alpha's
nuclear plants who died in an accident several months earlier. Unknown to the Alphans, a sphere of light is watching the Moon from space. Joy turns to astonishment when, in a matter of seconds, the newborn matures into a boy of six. Sue's son is
deaf and mute but very curious about Alpha's technology. Uncertain whether his accelerated growth was caused by their artificial environment or is the result of an external force, the Alphans do their best to raise the boy, whom they name Jackie. Professor Bergman and Captain Carter take a particular interest in his upbringing, with Carter becoming a
father-like figure to him. Commander Koenig, however, knows there is far more to Jackie than meets the eye. Sue is terrified of her child and is kept sedated in Medical. Jackie develops a
hypnotic power that enables him to control the Alphans' actions. Meanwhile, the sphere of light transforms into a spacecraft and approaches the Moon. It is soon joined by several other craft, and they all hold position over Alpha. Carter launches the Eagle squadron to defend the base, but the spacecraft fire energy beams that render the pilots unconscious and force the Eagles to land. Jackie becomes unresponsive and is rushed to Medical, where he is found to be undergoing another rapid growth spurt. Koenig takes a team out onto the lunar surface to shoot down the invaders using high-powered laser weapons. In Medical, Jackie is now an adult. Finding his voice, he introduces himself as Jarak, the new commander of Moonbase Alpha. Using his
psychic abilities, he forces Dr Russell to call Koenig, ordering him to abort the laser attack. He murders Sue and immediately resurrects her as his consort, Rena. After disarming Alpha's security guards, Jarak and Rena explain their presence on the Moon. They are fugitives from a race of beings who can take over the bodies of other species at moments of birth and death. Having objected to their society's imposed genetic conformity, they are leading 120 of their kind in search of a people to hide in. The Alphans will make ideal hosts. Combining their powers, Jarak and Rena begin choking the Alphans to death so their compatriots in the spaceships above can claim their new home. They are interrupted when a battlecruiser belonging to their pursuers arrives and shoots down the ships. Having lost their psychic grip on the Alphans, Jarak and Rena seal themselves in Medical. Jarak requests
political asylum, but his radio conversation with Koenig is cut short when the cruiser projects a beam of light at Alpha. Koenig and the other Alphans break in to find Sue tearfully cradling the newborn Jackie. There is no sign of Jarak and Rena. Russell supposes they surrendered, but Koenig suspects they were taken by force. ==Regular cast==