On the way to show Brighton to Leela, the TARDIS lands on the island of Fang Rock off the south coast of England in the early years of the 20th Century. Noticing that the lighthouse isn't functioning properly, the Fourth Doctor decides to investigate, as well as to ask for directions as the TARDIS seems to have gotten 'lost in the fog'. Upon arrival at the lighthouse, and after introducing themselves, the Doctor discovers the dead body of one of the keepers, Ben. The other two keepers, old superstitious Reuben and the keen young Vince Hawkins, report that a light fell from the sky near the island. They also explain the electricity flow to the lamp on the lighthouse has become erratic and the Doctor deduces something is feeding on the flow. Reuben does not help matters with his constant references to the mythical Beast of Fang Rock which reputedly once terrorised the lighthouse. As the Doctor and Leela explore, something moves Ben’s body out of the lighthouse and onto the island, and they witness a curious electric crackling which seems to have killed fish nearby.
The loss of the electric light due to the unexplained draining of power from the generators causes a luxury yacht to crash on to Fang Rock. The four survivors are brought to the lighthouse: the bosun Harker; an MP named Colonel James Skinsale; the owner, Lord Palmerdale; and his highly strung secretary Adelaide Lessage. Over time it emerges Palmerdale has bought government secrets from Skinsale and was desperate to reach the stock exchange to make a killing – hence the reason the ship was travelling at such a pace.
Harker and the Doctor retrieve Ben’s body and the Time Lord deduces it has been used as an anatomy lesson for an alien life-form. He determines that their best protection is to secure the lighthouse to keep the creature out. Reuben then disappears for a time and then reappears a changed man, which the others put down to shock. But the pattern of death now speeds up. Palmerdale is killed in the lamp room by a glowing alien presence on the outside of the lighthouse, and then Harker is killed when Reuben corners him in the boiler room. From the alien light emanating from Reuben it is clear he has become possessed or transformed by the alien creature. The Doctor finds Harker’s body and then Reuben’s own – the latter cold for some time – which means the creature in Reuben’s form has chameleonic properties.
The creature now stalks down and kills the others in the lighthouse. Adelaide dies first, then Vince. With its presence now revealed, the alien among them sheds its disguise: it is a Rutan, a chameleonic life form, whose scout ship crash landed in the sea and is trying to summon its mother ship. The Rutan ship is seemingly unstoppable, but the Doctor, Leela and Skinsale come up with a plan. First they kill the Rutan Scout — but not before it kills Skinsale — and then the Doctor uses Palmerdale’s diamonds as a focus for a light beam, and convert the lighthouse into a high-energy laser by which the Doctor destroys the Rutan mother ship. The blinding flash even turns Leela’s eyes from brown to blue. The Doctor quotes Wilfrid Gibson's poem Flannan Isle as they take their leave.
Doctor Tom Baker (Fourth Doctor)
Companion Louise Jameson (Leela)
John Abbott — Vince Hawkins
Colin Douglas — Reuben
Ralph Watson — Ben
Sean Caffrey — Lord Palmerdale
Alan Rowe — Colonel Skinsale
Annette Woollett — Adelaide
Rio Fanning — Harker
Writer Terrance Dicks
Director Paddy Russell
Script editor Robert Holmes
Producer Graham Williams
Executive producer(s) None
Production code 4V
Series Season 15
Length 4 episodes, 25 minutes each
Originally broadcast September 3–September 24, 1977