As dusk gathers over the hills and glens of Scotland, Captain Britain soars across the dark waters of Loch Ness. His head turns, as a rumbling growl below echoes eerily across the loch… and something vast and unspeakable is emerging from the murky depths.
Suddenly - with lightning speed - the monster strikes. Savage jaws close on its victim. Within scant seconds, a lifeless body falls from the monster’s jaws. Lifeless indeed: an android molded in the image of Captain Britain. A shadowed man announces, with a heavy Scottish accent, that all is just as the alien planned… the creature will kill swiftly when the time comes.
The next day, science student Brian Braddock and a group from Thames University arrive at an inn near Loch Ness. Does he believe the monster of Loch Ness really exists? Courtney Ross asks Brian, who replies that they’ve come to find out. A wishy-washy answer, but then you might expect Brian to dodge the question, Jacko Tanner mocks. He prefers to keep an open mind, Brian replies, till they probe the Loch with their new sonar-scanner. If Brian reckoned they’d have a real chance of finding Nessie, he wouldn’t venture within a mile of the loch, Jacko goads him, until Courtney tells him to shut up.
In his room, Brian ponders the question of the monster’ existence. He’s been so busy working on the bathysphere and scanner he had scarcely time to think about te monster itself.
At that moment, the innkeeper, McNab, is in mental contact with an alien, who informs him that he can sense the power emanating from Brian’s amulet. He gives his servant an order and, reluctantly, McNab obeys. He hates the though of harming a mere lad, but only a fool would refuse such a reward as the alien promises. McNab enters Brian’s room, bringing him a glass of warm milk.
Brian thanks him and drinks, noticing an odd aftertaste. Moments later, his head is spinning. He realizes he’s been drugged. He rubs the amulet and hopes that Captain Britain’s strength will help him fight off the drug. Instantly, he is transformed and thanks Merlyn, for he is shaking off the effect of that nighcap. He reasons that McNab had no reason to do him in, meaning he was following orders… but whose? He transforms back and pretends to be asleep, as McNab opens the Door.
Hours later, in the morning, the Thames University bathysphere is readied for its descent. Only Brian Braddock is missing, leading Jacko to announce that he predicted their brilliant young boffin would chicken out. Slowly, the undersea probecraft is lowered into the loch and Courtney switches on the sonar and sees something on the scope. Jacko sees something far more frightening - namely a leak. Water rushes in through a hole and Jacko signals the others to haul them up, while Courtney see the monster coming closer.
Above, the other students try to hoist the bathysphere up, while the alien telepathically tells McNab that the loch probe is negated. Has he eliminated the “dangerous one?” Mc Nab replies that he dumped his body in the marsh last night.
Said body is very much alive and Brian sees that the bathysphere’s cable has snapped. He changes to Captain Britain, takes a deep breath and dives down into the murky depths, until he reaches the bottom and the bathysphere. Thinking about Courtney depending on him, he takes hold of the bathysphere cable and actually manages to lift it.
Cheers fill the air, as the anxious crowd at the lochside see Captain Britain surface with his precious burden. Lowering the bathysphere on the ground, he dives back while the alien and McNab wonders how he survived. Down in the loch, Cap finally comes face to face with the huge reptile-like creature. Seeing the hero, it turns to attack.
Cap wants to return to the surface and get a camera… to prove to the world that the Loch Ness monster really exists, but, before he can do so, his attacker shoots out a cloud of ink, trapping the hero in a world of hostile darkness. While he cannot get his bearings, the monster’s jaws open - it intends to swallow him whole. Refusing to panic, Cap focuses on his sceptre making it expand. He puts it between the monster’s open jaws, keeping it from closing its jaw. And so, he escapes from the monster intent on returning to the surface before he can no longer hold his breath.
Suddenly, he hears a whirrling sound and sees robots coming out of the monster. It’s a machine! The robots pursue him with blasters and manage to hit him. And, as the stunned figure of Captain Britain sags limply into the water, more robots jet out of the hug monster-sub, bearing an enormous net. They capture the stricken superhero and tow him back into the deadly machine’s monstrous maw.