The Moon encounters a cloud composed of unknown elements. Commander Koenig, Captain Carter and scores of others are struck down by a mysterious ailment that causes depression, weariness and indecision. Dr Russell is unable to devise a treatment. Within days, only a few dozen healthy individuals remain. An Eagle is launched to collect particles of the cloud for analysis. It disappears, only to return three days later without its pilot. While searching the Eagle, Alpha's security personnel are attacked by a stowaway: a large, furred, frog-headed alien creature. Invulnerable to conventional weapons, it kills the entire team except Controller Verdeschi and astronaut Fraser, who barely escape. The voice of the beings inside the cloud transmits a message, stating that they need Alpha's
life support system to survive. They have sent the creature to capture it. As the creature approaches Command Center, Verdeschi evacuates everyone to the medical and life-support control sections, then initiates a base-wide lockdown. Using himself as bait, he lures the creature into a
vacuum chamber in an attempt to asphyxiate it. However, it survives and attacks him. Maya comes to Verdeschi's rescue. To distract the creature, she attempts to shapeshift into its double, but finds its physiology impossible to imitate and is forced to transform into a different animal. Verdeschi breaks free and uses fire-fighting equipment to push the creature back. Verdeschi and Maya retreat to Medical, where they find that Russell has contracted the disease. She recommends using an injector gun to give the creature a massive dose of ionethermyecin, a powerful anaesthetic. Verdeschi and Maya broadcast over Alpha's public address system to divert the creature to a
hydroponics chamber, where they plan to suffocate it with
chlorine. Verdeschi also contacts Fraser and Sandra Benes and orders them to fortify the approach to Life Support by setting up a barricade of high-voltage electrical cables. With Maya in the form of a chlorine-breathing alien, the creature is tricked into the chlorine-filled room and injected with the anaesthetic. Neither the gas nor the anaesthetic has any effect, and Verdeschi and Maya are forced to flee. Verdeschi realises that instead of attacking the creature, they could attack the beings controlling it. Activating one of Alpha's laser batteries, they fire at the cloud – which fragments, but quickly re-forms. As the alien voice laughs mockingly, Verdeschi and Maya rush to Life Support to make their
last stand. They admit their mutual love and kiss. Nearing Life Support, the creature is repeatedly repelled by the barrier's electric shocks but simply keeps coming. Maya realises that it is not an organic being, but a robot. Finally breaking through, the robot knocks Fraser unconscious, injures Verdeschi, and removes the life support control core, causing all vital systems to shut down. Spotting an opening in the robot's ear, Maya shapeshifts into a bee, flies inside the robot's head and crawls around its insides,
shorting out its circuits. The robot seizes up and keels over, defeated. Verdeschi replaces the core and life support is restored. With the destruction of its agent, the cloud vanishes, as does the epidemic that came with it. ==Regular cast==