On the lonely English coast sits a light house, its beam shining into the night. A purple ball of fire falls into the ocean beyond, creating a glow deep in the water.

A young man -- Vince -- watches the glow through his telescope. He calls the elderly lighthouse keeper Reuben over and tells him about the light but they see nothing now. A middle-aged man named Ben joins them. Reuben suggests it was a meteor; Ben jokes that they're looking for mermaids. As long as it's not a hazard to navigation, they don't need to worry about it. The older men go downstairs for supper.

But something in the ocean watches them through a green haze.

Over dinner, Reuben laments the disappearance of oil in lighthouses. He and Ben argue about whether electricity is a better system. Ben insists it is both cleaner and cheaper. They are interrupted by Vince calling down to tell them that a fog has come up. Ben is shocked -- there were no signs of it earlier. He goes up to check it out -- Vince is new after all and just learning the job.

A second light appears in the foggy mist -- the TARDIS materializing.

Vince has never seen a fog come in so fast. It's gotten icy cold as well. Vince tells the other lighthouse keepers the fog rolled in from where the meteor landed. Ben makes him turn on the foghorn while Reuben says the fog isn't unnatural. He comments that oil lamps are better in fog but Ben replies that electricity is more reliable. As he says this, the lamp goes out, much to Reuben's amusement.

Leela emerges from the TARDIS, clad in a fancy dress. She tells the Doctor she doesn't like Brighton. The Doctor tells her they aren't in Brighton. He's not sure where they are; the TARDIS has gotten lost in the fog. He then notices the darkened lighthouse.

As Ben examines the lighthouse engine, the lights flicker back on. Puzzled, he heads back upstairs. Shortly after, the door opens and something green enters.

Ben find Vince in the crew room, putting on a sweater -- it's gotten freezing cold in the lamp room. Ben tells him the machine came back on by itself.

Leela notices the light has come back on and the Doctor decides to stop in the lighthouse and get directions. They hear the foghorn ring out. Leela begins to sense that something is wrong.

As Vince brings a heavy coat to Reuben, the light goes out again. Ben runs down once more to check the generator. He opens the door the coal hole and something green emerges. His scream is drowned out by the foghorn.

Reueben comments that in 30 years, he's always known when a fog was coming -- until tonight. Vince goes down to check on Ben.

The Doctor and Leela reach the generator room. He wonders why the machine isn't working but decides to speak to the crew before he fixes it. He finds Vince on the stairs and explains that their ship got lost in the fog. Vince takes them to the crew room and tells then Ben is working to fix the generator. But the Doctor hasn't seen him. Vince wonders if they just missed him in the fog but Leela assures him she would have sensed him. The Doctor decides to help repair the generator after all.

Vince is awkward and shy with Leela, telling her he gets so bored on Fang Rock, he sometimes talks to seals. When she mentions that she could use some dry clothes, he tells her they don't have clothes for a lady. She tells him his clothes will do fine and begins undressing. An alarmed Vince covers his eyes and leaves. Leela just shrugs it off.

The Doctor look for Ben. The lights come back on and the Doctor inspects the generator. Vince comes down to thank him for fixing it and the Doctor tells him he's found Ben's body -- killed by an electric shock. Vince is devastated and goes to tell Reuben. Leela asks the Doctor if he thinks the generator killed him and the Doctor shakes his head. He checks the coal hole but doesn't find anything. Leela wonders if a sea creature killed him but the Doctor is puzzled how it could drain the electricity and not leave any marks. He then finds Ben's lantern -- crushed flat.

Reueben crosses himself when he hears of Ben's death. He can't believe it -- Ben knew the machine so well. He is suspicious of the Doctor and Leela -- perhaps they are foreign spies. The trouble started when they arrived, after all. After promising to send for a relief boat, he goes down to take a look at them. He comments that if Ben was killed by the machine, there will be anger in his soul.

The Doctor is examining the telegraph when Reuben arrives. The keeper barks at him to leave it alone. He takes a funeral shroud from Leela, who is using it as a napkin. The Doctor asks Reuben if he thinks they had anything to do with Ben's death but the grim man only replies, "I know what I know . . . and what I think". He leaves to tend to Ben and Leela asks if the creature will return. She wants to hunt it but the Doctor doesn't "fancy playing tag in the fog" with something that can crush lanterns and kill with electricity. But the girl is not persuaded and, as Reuben makes Ben's body decent, she sneaks past him and out into the fog.

The Doctor asks Vince about the fireball. Vince explains that it arrived at dusk about a mile or two offshore. The fog and cold followed at once.

Leela stalks the shore with her knife poised. She finds a pool of dead fish and hears an electric crackling in the distance.

The Doctor is telling Vince about Pharos when Reuben returns and tells the young man to get some sleep. He merely glares at the Doctor.

Vince hears an electric crackling as he descends the steps.

Leela continues to hunt as the green creature watches her.

Vince stop in the generator room and finds Ben's shroud empty. He calls up and tells an incredulous Reuben that Ben is walking. The Doctor then spots a light out on the ocean.

Leela assures Vince that the dead can't walk. She thinks something on the rocks dragged him out. Reuben calls down and tells them a ship is coming in very fast and will strike on the rocks.

Reuben, the Doctor and Vince race out into the fog to help the survivors of the crash, leaving Leela to keep the foghorn going. The Doctor stops to pick up a rope and sees an electric discharge off the generator. He hesitates but Reuben calls out for him to follow.

Leela takes great delight in sounding the foghorn over and over. After a while, the light comes back on.

The three men wade through the fog with lanterns. Vince notices the light back on and Reuben laments that it wouldn't happen with oil. The Doctor nods that "it" needs electricity.

Leela, bored with the foghorn, goes to the balcony. She looks down and sees a green tentacled creature moving on the rocks.

Three survivors are brought to the lighthouse. A rich businessman named Palmerdale, an older dignified man named Skinsale and Palmerdale's young pretty assistant Adelaide. Palmerdale wants a brandy but Vince suggests hot soup instead. Reuben goes to the lamp room while the passengers take off their life jackets and huddle around the stove.

Leela tells the Doctor about the luminous creature. Reuben overhears as the Doctor advises her not to tell anyone. He then tells the Doctor he knows what they saw - the Beast of Fang Rock.

Palmerdale demands some clothes and brandy but Vince tell them alcohol is not allowed in the lighthouse. The Doctor returns and asks about Captain Harke. He is told Harker stayed behind to secure the boat. Skinsale explains that Harker's seamanship saved them all but the Doctor asks who got them on the rocks. Skinsale looks at Palmerdale significantly.

Skinsale asks the Doctor if he's in charge and the Time Lord replies with a grin that he's full of ideas. Vince goes to stoke the boiler and the Doctor sends Leela with him.

Adelaide asks why he's sending women to stoke boilers. The Doctor explains that a keeper was killed and Vince doesn't like going down there alone.

The Doctor is introduced to everyone. Palmerdale is a financier, Skinsale is a member of Thurley. Palmerdale is eager to get to South Hampton to get to the stock exchange. The Doctor tells him this is impossible and Skinsale laughs that maybe Palmerdale didn't win all he thought at the casino.

As Vince stokes, Leela hears something being dragged on the rocks. She sends Vince to get the Doctor and grabs a shovel as a weapon. But it is only Harker dragging a body inside. The Doctor comes down and examines the body -- all that's left of Ben. Harker laments what the seas does to a man but the Doctor says the sea didn't do it. He sends Harker away and closes the door.

Leela asks if the Beast of Fang Rock ate him but the Doctor says there is no such thing. Something did a post mortum on him to study human anatomy. He hides the body as Vince comes down. Vince is alarmed but Leela assures him again that the dead do not walk.

Palmerdale wants to set out for the mainland once Harker is rested. His companions are appalled and Harker outright refuses. Palmerdale threatens to fire him but Harker is unmoved. "Hang him from the yard arm, Henry, it's mutiny," quips Skinsale.

The Doctor tells Vince that Ben was just stunned. He walked out onto the rocks and was drowned. This seems to placate the young man. Leela asks the Doctor why he didn't tell Vince the truth and the Doctor explains that he doesn't know the truth.

Palmerdale is now raving that the lighthouse is responsible for the crash, and so they should get him to South Hampton. Skinsale and Harker tell him they would have hit no matter what. The Doctor returns and tells them to stay in the lighthouse. Palmerdale thinks the Doctor is crazy and Adelaide doesn't like Leela. Skinsale calls up to see if Adelaide can get a room and Reuben tells them to try the bunks in the sleeping quarters.

Vince sees lights in the fog and Reuben opines that it's the Doctor and Leela. He warned them about the Beast of Fang Rock. Vince is skeptical but Reuben tells him Leela saw it. He explains that the last time it was seen was 80 years ago -- and it left two men dead, the other mad.

The Doctor and Leela creep through the fog. The Doctor puts a compass down where Leela saw the creature and watches as the needle swings around in a powerful electrical feld. Leela shows him the dead fish. The Doctor isn't sure what's out there but it is desperate and cunning. As they leave, a green glow follows.

Skinsale comes back, having put Adelaide to bed. It becomes clear that he gave some information on stocks to Palmerdale, who can't take advantage of it. Palmerdale threatens to expose him anyway but Skinsale points out that without proof, the "jumped-up moneygrubber" wouldn't be believed over a gentleman.

The Doctor and Leela return to the lighthouse. The Doctor now thinks Vince's fireball was a spacecraft. Some creature has been scouting them -- something that needs electricity. He locks the lighthouse door and explains that it created the fog to isolate them and will soon attempt to destroy them. Leela is ready to fight it but the Doctor points out the others will think they're mad if they start talking about aliens.

Palmerdale scribbles out a message as Skinsale sleeps. He wakes Harker and asks him to telegraph it to his brokers. But Harker remembers how desperate he was to get London and how many sailors died when the ship struck. He assaults Palmerdale and Skinsale and the Doctor have to pry them apart.

The Doctor then says, with a grin, "This lighthouse is under attack and by morning we might all be dead. Anyone interested?"

The green thing closes in on the tower.

Reuben comes to the lamp room and tells Vince to stoke the boiler again but the young man is now concerned about the beast and the parallels between what's happening to them. Vince is shaking and Rueben kindly offer to stoke the boiler.

The Doctor tells the crew that no one is to leave. Reuben arrives and tells them about the Beast of Fang Rock, which makes Palmerdale laugh. Leela threatens to cut out his heart if he doesn't listen to the Doctor. But then she notices that it's getting cold. Skinsale doesn't sense anything but the Doctor assures them that her senses are very acute. The lights dim as Adelaide arrives.

Reuben notices the lights fade and shakes his head. He crosses hismelf and opens the coal hole.

The Doctor and Leela rush down the the boiler room. Reuben is gone, the door open. Leela thinks the creature has taken him and the Doctor rushes out into the fog.

Adelaide is in tears over the scream. She should have listened to Mrs. Nethercutt, her astrologer, who predicted disaster. Harker goes down to help but Palmerdale objects that they should stick together. Skinsale's reply, "What, on the principle that it may satisfy its appetite before it reaches you?" provokes more tears from Adelaide.

Harker is about to go out when Reuben comes out of the coal hole. He passes in a daze, simply saying, "Leave me be!" and goes upstairs. Harker calls out into the night for the Doctor.

Reuben stops outside the crew room as the lights go back on. Palmerdale hears him on the stairs and Skinsale goes out to check. He suggests Adelaide go back to bed but she's too scared.

Harker stokes the boiler as the Doctor and Leela come back in. The Doctor thinks the creature killed Reuben but Harker tells them he's returned. The Doctor touches the generator and gets a spark. He tries to calculate the size of the creature but then realizes he can just ask Reuben how big it is. He leaves Harker to secure the door.

Palmerdale asks Skinsale to radio his instructions to his broker but the nobleman refuses -- he'd be ruined. Palmerdale heads up to the lamp room and Skinsale follows him, suspicious of his motives.

The Doctor knocks on Reuben's door but the keeper is unresponsive. He starts glowing green. The Doctor wonders if it's shock. He sends Leela down to make sure the boiler stays stoked.

Palmerdale talks to Vince about making some extra money. As Skinsale listens in, he asks him to use the telegraph to call his brokers, offering him a hundred pounds. Skinsale heads down as Vince takes the money.

The glowing creature scales the tower as Palmerdale gives Vince his instructions. He then sneaks out to the balcony as the Doctor arrives.

Harker has secured the door and Leela takes the sledge hammer from him, hefting its weight thoughtfully.

Vince wonders if the Beast of Fang Rock has returned. The Doctor dismisses the story and the parallels pointing out that there are eight people in the lighthouse who can defend against the creature. As they talk, Palmerdale shivers on the balcony. Suddenly, he is enveloped by a green electric glow.

Adelaide's fear has given way to anger as she rebukes Skinsale for the nasty things he's saying about her employer. She decides to go up and tell Palmerdale everything. After she leaves, Skinsale smiles and approaches the telegraph.

Leela threatens to knock down Reuben's door if he doesn't answer. Inside, Reuben reappears in a green glow.

The Doctor tells Vince they will stay in the lighthouse until morning. But when he hears crashes below he rushes down and stops Leela -- telling her they'll wait until Reuben is ready to speak. Adeliade comes up looking for Palmerdale but the Doctor forcefully sends her back to the crew room.

Vince comes out to the balcony but can't find Palmerdale. He is about to call down and alert everyone but stops a moment to burn the bribe money, worried they'll think he killed the financier for it.

The Doctor tells Leela to summon everyone to the crew room. Skinsale is skeptical as the Doctor tells him there is an alien out in the fog. Leela returns and reports she can't find Palmerdale. Just then, Vince phones down that Palmerdale has fallen from the tower. Adelaide explodes in tears and Leela slaps her. Skinsale wants to go out to find Palmerdale but the Doctor is hestitant, making the former realize the Doctor is serious about the alien.

The Doctor, Harker and Skinsale go out, watched by Reuben. Adelaide sobs and tells Leela about her astrologer's predictions, but Leela dismisses this, saying she now believes in science.

The Doctor carries Palmerdale's body in and makes Harker secure the door. When they bring the body into the crew room, Adelaide begins weeping again. She accuses Skinsale of killing him. Skinsale admits he overheard Palmerdale trying to bribe Vince. The Doctor realizes Skinsale then smashed the telegraph to protect his honor, isolating them from the mainland.

Skinsale tells Adelaide he didn't kill Palmerdale -- perhaps Harker did. The Doctor comments that he wishes Skinsale had killed Palmerdale -- it would make everything simpler. But Palmerdale was dead of electrocution before he hit the ground. Skinsale realizes this means the alien creature can scale sheer walls.

Leela calls down and reports the boiler pressure has fallen. The Doctor realizes that something has happened to Harker. He and Leela rush down and find Harker electrocuted. The Doctor sends everyone back to the crew room.

Vince is in the light room when Reuben approaches silently. Vince backs away but Rueben grabs and electrocutes the terrified keeper.

Leela stokes the boiler as the Doctor ponders his next move. She's not concerned, believing the Doctor will easily dispose of the creature. Leela suggests they pretend they think Reuben is still himself to get close enough to kill it. But the Doctor points out it would kill them at close range. He finds a strange device on the generator -- a power relay. It must be powering a distress beacon somewhere. He tells Leela to get everyone to the lamp room -- it's the easiest place to defend.

Leela goes to the crewroom and tells Skinsale and Adelaide that Harker is dead and the creature is inside. They must fight for their lives. Adelaide faints.

The Doctor searches Reuben's room. He hears the keeper approaching and hides in the bunk. Reuben searches for the Doctor but can't find the Time Lord, who is hanging from the windowsill.

Skinsale revives Adelaide. Leela urges them to head up to the lamp room but hears the heavy tread of Reuben on the stairs.

The Doctor climbs back in through the window. He sees something above the shutter and pulls down a piece of equipment.

Reuben quickly kills Adelaide. Leela throws her knife at him but it simply passes through the glowing Reuben. But evading the knife slows him down enough for her and Skinsale to run. They encounter the Doctor on the stairs. He tells them to find maroon flares in the lamp room locker and scatter the powder on the stairs.

The Doctor sits on the stairs and asks Reuben if it is difficult to maintain his human form. Reuben replies that it is no longer necessary and, with a green glow, changes into a spherical craeture.

Leela and Skinsale find Vince's body. They then open the locker and take out the flares.

The Doctor recognizes the creature as a Rutan. It tells him it's a scout with a new metamorphosis technique. The Doctor taunts it about losing the war with the Sontarans but the Rutan is defiant. They intend to invade Earth and make it a strategic outpost. Although this will eventually mean the planet is bombarded from space, the Rutan doesn't care.

The Doctor tells the Rutan it has failed. The Rutan replies that it is sending a signal to its fleet and the Doctor tosses the defunct transmitter at it. The annoyed Rutan says that the mother ship will home in on its primary signal but the Doctor has disabled that as well. The Rutan angrily crackles and tells him that it was transmitting long enough. It then follows him up the stairs, ready to finish off the survivors.

When they reach the top of the tower, the Doctor takes a fuse from Skinsale and tosses it onto the powder, setting off an explosion. The creature screams in pain and retreats. The Doctor tells them to find more gunpowder -- flames are painful to the Rutan.

Skinsale find a rocket-firing mortar. They start tossing random odds and ends into the rocket launcher as the Doctor worries about the Rutan fleet. But the time the Rutans and Sontarans finish with Earth, it will be a cinder. But if they can somehow knock out the mother ship, it might scare the Rutans off. He realizes that the Rutan ship is crystalline and a simple carbon laser could destroy it if its shields are off - which they would be when landing on a primitive planet.

Leela suggests using the lighthouse beam but the Doctor would need a diamond to focus the light. Skinsale remember that Palmerdale always carried diamonds with him.

They set up the mortar as the wounded creature begins to ascend the stairs again.

The Doctor and Skinsale head down to the crewroom. The find the diamonds as the creature closes in. The Doctor picks one out and tosses the remainder on the floor. Skinsale stops to pick them up but is caught and killed by the Rutan.

The Doctor races up the stairs with the Rutan in hot pursuit. Leela ignites the mortar as they reach the top and the creature screams from the blast. Leela goes after the mortally wounded Rutan as the Doctor turns off the light.

Leela gloats over the dying Rutan as its desicated remains slither down the stairs. The Rutan tells her their triumph will be short-lived -- the mother ship will destroy them. It dies praising the Rutan Empire. When she returns to the lamp room, the Doctor chides her for gloating over the Rutan.

An orange ball of fire descends and hovers over the island. The Doctor fixes a diamond in the searchlight. As the mothership approaches, he points the light at it and switches on. He cautions Leela not to look back and they race down the stairs, Leela pausing to retrieve her knife.

They scramble out onto the rocks as the glow brightens. They take shelter but Leela looks up at the last minute just as the Rutan ship explodes.

Leela asks the Doctor to kill her -- the flash has blinded her. He takes her knife but only laughs, telling her the effect will pass. She blinks a few times and the Doctor swims into view. He tells her that her eyes have turned blue.