On the way to Brighton, the TARDIS lands on the island of Fang Rock, near the lighthouse, about 1902 or 1905. Poor Leela must think Earth is a miserable place—first Victorian London, then this fog-shrouded rock in the sea. She has to wear clothes here too. Noticing that the lighthouse isn’t working right, the Doctor decides to investigate, if only to ask directions. Upon their arrival, they discover the dead body of one of the keepers, Ben (Ralph Watson). Superstitious old salt Reuben (Colin Douglas) and young Vince Hawkins (John Abbott) report that a light from the sky fell near the island. The electric light hasn’t worked right since. As the Doctor and Leela explore, Ben’s body is moved out of the lighthouse, and they find electrocuted fish in the tidal pools.
The loss of the light causes a yacht to ground on Fang Rock. The four survivors are brought to the lighthouse: the ship’s bosun Harker (Rio Fanning), Colonel James Skinsale (Alan Rowe), the arrogant Lord Palmerdale (Sean Caffrey), and his over-wrought hysterical secretary Adelaide Lessage (Annette Woollett). Leela is tempted to stick a knife in her. It turns out the ship was sailing too fast for the conditions because Palmerdale and Skinsale were on their way to London to make a killing on the market with some stolen government information.
Harker and the Doctor retrieve Ben’s body and the Doctor recognizes the damage as an alien anatomy study. They lock the doors to keep the creature out. Reuben disappears for a while and comes back in a dazed state. But more people die. Palmerdale dies in the lamp room, then Harker is killed by Reuben in the boiler room. It’s an Old Dark Lighthouse story, and suitably atmospheric, with the fog, the flickering lights, and the lugubrious foghorn. An alien light is emanating from Reuben, who is clearly possessed. Then the Doctor finds his long-cold body, proving that what appears to be Reuben is an alien in his shape. He thought he was locking it out, but he was locking it in.
The creature kills Vince, and Adelaide, shutting her up for good. The alien appears in its true glowing, green, tentacled form and the Doctor recognizes it as a Rutan, the ancient enemy of the Sontarans in their forever war. Its scout ship crashed in the sea (good thing it’s amphibious) and it is trying to use the lighthouse to signal its mother ship. The Rutans are losing the war and would like Earth as a base. Funny, how this small planet in an outer arm of the galaxy is so strategic. Unfortunately, once the Sontarans find Earth, they will destroy it. The Doctor modifies a weapon to destroy the Rutan. They use the diamonds they found on Palmerdale’s body to focus the lighthouse beam and turn it into a laser (who knew?). Skinsale is killed, but the Doctor destroys the Rutan mothership. Watching the explosion, Leela’s eyes change from brown to blue, because Louise Jameson was sick of wearing contacts.
Filming this story, Louis Jameson finally put her foot down about being upstaged by Tom Baker and gained his respect. I think Leela was rubbing off on her. The story was praised as tight, claustrophobic, atmospheric, and scary, despite the unconvincing Rutan. On 22 November 1987, a re-broadcast of one episode was hijacked and interrupted for 90 seconds by something pretending to be a soft-core Max Headroom. The perpetrators have never been identified.