Alien troops invade Moonbase Alpha, seeking Maya's brain to extend their abilities and obtain immortality; guest Ann Firbank.
Koenig must convince a powerful alien bent on destroying all visitors to set the Alphans free.
Alien cat-women with electric whips pursue Koenig in a deadly cat-and-mouse hunt; guest Hildegard Neil.
An evil Psychon (Lee Montague) tricks Maya (Catherine Schell) into teaching him the secrets of molecular transformation.
A deranged crewman's (Ken Hutchinson) actions threaten to destroy the Alphans; guest Carolyn Seymour.
Koenig and Maya fight to save Moonbase Alpha from deadly aliens; guests Stuart Damon, Patrick Westwood.
Aliens disguised as friends from Earth trick all but Koenig; guests Stuart Damon, Patrick Westwood.
A cosmic cloud gives a crew member (Deborah Fallander) paranormal abilities; guest Jess Conrad.
Even Maya's (Catherine Schell) powers fail against a seemingly invincible creature that incapacitates the crew.
An ancient being (Stuart Wilson) tries to alter his race's fate by sacrificing the Alphans; guest Lynne Frederick.
Delirious Maya (Catherine Schell) endangers herself and the crew by transforming herself into an array of fearsome creatures.
A miner (James Laurenson) has vivid visions of his wife's (Pamela Stephenson) demise and Moonbase Alpha's destruction.
The Alphans discover the cocoons of deadly chlorine-breathing monsters (Ina Skriver, Sarah Douglas).
Koenig is trapped in a crystalline asteroid cave, where his double emerges and returns to Moonbase Alpha.
A magical trader (Willoughby Goddard) agrees to send the Alphans back to Earth in exchange for Maya (Catherine Schell).
A living rock in desperate need of water enslaves Tony and paralyzes Helena; guest Patrick Mower.
As punishment for destroying vegetation, Maya and Koenig battle three aliens (David Jackson, Godfrey James, Roy Marsden).
Cryogenically preserved beings (John Standing, Michael Gallagher) infect the crew with a deadly disease.
A godlike being (Guy Rolfe) offers the Alphans a chance to live on a planet resembling Eden.
A willful computer forces Helena and Koenig to accompany it to a poisonous planet; guest Bernard Cribbins.
A race of superior robots forces Helena and Tony to teach it human emotions; guests Billie Whitelaw, Leigh Lawson.
Helena, John and Alan travel back in time to plague-infested Scotland; guests Freddie Jones, Isla Blair.
Bizarre capsules circling Moonbase Alpha contain ancient killers in suspended animation; guests Peter Duncan, Stacy Dorning.
Koenig and Helena appeal to the daughter of an alien ruler planning to use the Alphans as fuel for a biological computer.
Koenig arranges a cease-fire between two warring planets when Moonbase Alpha is caught in their fire; guest Caroline Mortimer.
Koenig must decide if the crew should evacuate to an abandoned planet or escape when Moonbase Alpha's orbit ceases.
A botanist's (Giancarlo Prette) experiments in communication between plants and man bring bizarre, frightening results.
Koenig decides to use explosives to prevent a collision with an unseen intelligence; guests Shane Rimmer, Carla Romanelli.
A verbal communication from the planet Ariel destroys the crew's hopes of settling on a planet apparently similar to Earth.
Koenig becomes enamored of a beautiful woman when an anthropologist (Peter Cushing) brings him to a paradisiacal world.
Koenig must prevent an alien force on an obsolete mission from destroying Moonbase Alpha and its occupants.
A machine named Gwent imprisons Helena and Koenig on a spacecraft to serve it for the rest of their lives.
Past and future overlap as a sudden acceleration creates a duplicate moon complete with replicas of the crew.
Several of the crew become Cro-Magnons after being caught in a time warp on a newly discovered planet.
Simmonds (Roy Dotrice) forces Koenig (Martin Landau) to let him take the place of a dead alien on an Earthbound ship.
Helena's (Barbara Bain) missing husband (Richard Johnson) appears to warn the crew away from an antimatter planet.
An immortal alien (Peter Bowles) released from exile by Koenig (Martin Landau) destroys all life it encounters.
A beautiful woman (Catherine Schell) convinces all but Koenig to join her on a seemingly paradisiacal planet.
Bergman (Barry Morse) brainstorms to stop Moonbase Alpha from being sucked into a deadly black sun; guest Paul Jones.
The crew answers a ship's distress signal and discovers its occupants employing a gruesome survival technique.
Cellini (Gianni Garko) tries to vindicate himself by proving the existence of a terrifying, tentacled monster.
Cynthia's son (Julian Glover) turns out to be an alien seeking human bodies for fleeing fugitive space travelers to inhabit.
Revenge-seeking aliens complicate matters as the crew tries to incapacitate a deadly ship containing valuable information.
Two inhabitants (Brian Blessed, John Shrapnel) of an ice planet encourage Koenig to join them in immortality.
Koenig appeals aliens to allow the crew to settle on their planet after the aliens' attack leaves Moonbase Alpha inhabitable.
An ancient alien queen (Margaret Leighton) persuades Koenig to allow their moon to collide with her planet.
Colleagues die when Zoref (Ian McShane), possessed with a consuming need for heat, extracts it from their bodies.
An explosion disintegrates the moon's dark side and hurls it, along with 311 occupants, out of Earth's orbit and into space.