On Titan, two NTI geologists uncover an ancient alien laboratory hidden beneath the moon’s rugged surface. In a sealed chamber they discover an egg-like container that keeps an alien creature alive. When the creature finally emerges, it kills the two researchers, setting a dark and perilous tone for what follows. Two months later, the geologists’ spaceship crashes into the space station Concorde, orbiting Earth’s Moon, its pilot dead in the seat.
NTI dispatches a new vessel, the Shenandoah, to Titan. Its crew includes Captain Mike Davison [Stan Ivar], Susan Delambre [Marie Laurin], Jon Fennel [Robert Jaffe], Dr. Wendy H. Oliver [Annette McCarthy], David Perkins [Lyman Ward], and Beth Sladen [Wendy Schaal], joined by the taciturn security officer Melanie Bryce [Diane Salinger]. While in orbit, the crew picks up a distress signal from a rival ship, the West German Richter Dynamics. A sudden ground collapse drops the Shenandoah into a cavern, wrecking the landing, and radio communication fails. A search party is dispatched to contact the German vessel.
On the Richter ship, the explorers find the same egg-like container and the dead crew. The creature surfaces and attacks Delambre as she lags behind the others. Fennel falls into shock at the sight, and Bryce sedates him. Returning to their own ship, the Americans discover that Hofner, a West German crew member, has slipped aboard unnoticed. He claims his team was slain by the creature, which had been part of a galactic menagerie. Hofner proposes returning to his ship to retrieve explosives, but the Shenandoah’s crew are wary of the risk.
The creature’s undead victims are controlled by parasites. In the medbay, Fennel watches the undead Delambre through a porthole, follows her outside, and is paralyzed as she removes his helmet. He suffocates, and then she attaches a parasite to his head. Now under alien control, Fennel sends a transmission inviting his crewmates to the German ship. Hofner and Bryce head to fetch air tanks for the Shenandoah, while the rest of the crew treks to the Richter vessel. Hofner and Bryce stop by the menagerie on the way and are attacked by Delambre. The others discover Fennel’s head wound hidden by a bandage and learn that Oliver has been decapitated by the creature. Perkins shoots Fennel’s head with his pistol to end the threat.
Sladen encounters an infected Hofner and escapes, but she forgets her helmet in her hurry. Perkins spots her, opens the airlock, and she is drawn back inside. Hofner carries Sladen to lure the others, and a struggle ensues. Davison defeats Hofner by tearing out his parasite. The survivors—the captain, Sladen, and Perkins—devise a plan to electrify the creature using the Shenandoah’s fusion modules, a route that runs through engineering.
alarms rise as the creature sabotages the ship. Sladen and Davison set up an electrocution trap while Perkins monitors the creature’s movements from the computer room. The creature appears and seems to be electrocuted, but Davison soon learns that Sladen has been captured. He and Perkins race after her, finding her locked inside engineering. Consulting the blueprints, they discover another route into engineering. Perkins lures the creature away while Davison retrieves Sladen. He also finds one of Hofner’s bombs. The creature attacks Perkins in a final bid for control. As Perkins dies, he attaches the bomb to the creature and triggers a countdown so Davison can jettison it through the airlock. The creature reenters the ship, and Bryce arrives just in time to shoot it, triggering the blast that annihilates the monster. Davison is rescued by Bryce and Sladen, who tend to his wounds. The trio then leaves Titan aboard the West German ship, bringing the peril of their ordeal to a hard, but hopeful, conclusion.
