It was a dark and stormy night… and then everyone died in a lighthouse. Cozy!
Short Synopsis
The Doctor and Leela stumble into a lighthouse during a storm, only to find that its keepers are being picked off one by one by a shape-shifting alien. Spoiler: the alien is better at murder than the humans are at basic survival.
Extended Storyline
The TARDIS lands near a lighthouse on a foggy, storm-battered rock off the English coast. The Doctor and Leela seek shelter, only to discover that one of the lighthouse keepers has already died under “mysterious circumstances” (translation: death by special effect).
Soon enough, strange lights appear in the sky, the power keeps failing, and the keepers start muttering about “beasties” in the fog. Naturally, the Doctor deduces that an alien menace has landed nearby. Because when isn’t it an alien menace?
Enter the Rutan: a glowing green jellyfish with all the charisma of a lava lamp and the murderous tendencies of a slasher villain. Worse, it can shape-shift, which means everyone spends the story accusing everyone else of being an alien impostor — and they’re usually wrong. Leela, of course, is the only one advocating the sensible approach of stabbing first and asking questions later.
As if murderous blob-monsters weren’t enough, a boatload of upper-class twits gets shipwrecked on Fang Rock and immediately start complaining about the accommodations. These snobs then get systematically bumped off by the Rutan, who clearly doesn’t care for posh whining either. The body count climbs steadily, and the Doctor grows increasingly grim, while Leela grows increasingly impatient with people not stabbing the obvious suspect.
In the end, the Doctor lures the Rutan into the lighthouse lamp room, rigs up an impromptu energy weapon, and zaps it into oblivion. Victory! Except not really, because by this point literally everyone else is dead. The Doctor and Leela leave Fang Rock standing alone, the storm still raging, with nothing left behind but corpses and a really traumatized foghorn.