The TARDIS is shaking violently and sparks are flying from the console as the Doctor and Gisella hang on for dear life. He tells her that they are trapped in a temporal slipstream and reaches for the mallet. He gives the controls a mighty whack and order is restored at once. The journey continues peacefully until the dematerialization when the Doctor checks the coordinates with consternation. They have arrived in the Silver Devastation. The Agent has led them to an area of space where two galaxies once collided; now it is the source of myth and rumour about monsters and mutants.

Stepping out of the TARDIS they find that they are on the manicured lawns of a country estate. They see an old man who introduces himself as Ted the gardener. He tells them that the ‘others’ are at the house and asks to see their invitation. After looking at the Doctor’s psychic paper he points them on their way and wishes them luck, hoping that they are not the first to be killed. The Doctor demands to know what he means but Ted grows uncomfortably embarrassed and continues with his work. They continue up a lengthy drive to find an elegant country house. The Doctor is suspicious and tells Gisella that there is no way such a place can exist in the middle of a region of stellar destruction.

They are greeted at the door by an imperious butler who welcomes them to Devastation Hall. They are introduced to an old lady, Miss Amelia Birch, who tells them not to mind Stokes the butler. She is surprised that they are not the Doctor Black and Nurse White she was expecting but welcomes them anyway. The Doctor asks which part of Surrey or Kent they are in so Miss Birch leads them down to the edge of the estate where the ground falls away sharply and they find themselves staring out at stars and distant nebulae. She tells them that they are within an environmental bubble in the Silver Devastation. As they walk back to the house the Doctor explains to Gisella that they are in an enclosed micro-climate and that the setting sun is actually an energy field in a containment sphere.

As they reach the house a sleek spaceship lands on the lawn and a handsome pilot leaps out. He is Horatio Hamilton, a dashing hero in every sense, and he says he has made it up from Altair 59 in less than a day. Despite the Doctor recognising Hamilton’s name as the winner of the Mars-Centauri Grand Prix he privately muses on the impossibility of getting to the Silver Devastation so quickly even in such a fast ship. Miss Birch tells the pilot that he can’t park on the lawn and must take his craft to the spaceport at the back. As Hamilton moves his ship the Doctor and Gisella are led back to the house by their hostess who tells them that there will be cocktails in the drawing room before the killing begins.

The newcomers are introduced to the other guests: General Korch (retired), a dazzling beauty called Miss Scarlett Plume and a bored young man called Lenny Sprang. He apparently won his visit to Devastation Hall in a hypernet competition. Hamilton bursts in just then, leading an elderly man, Professor Doofus. Gisella asks what they are all there for and Miss Birch tells them that they have come to play The Game of Death.

The Doctor and Gisella return to the TARDIS to put on evening dress for a formal dinner. They walk back through the moonlight to the house (the moon is actually the sun turned down) discussing the Game of Death. Gisella thinks that there is something else in the garden watching them but she won’t let the doctor investigate, even though it might be the Darksmith Agent that they followed to Devastation Hall. They meet Ted the Gardner again. He has just emerged from the wine cellar where he tells them that the asteroid’s environmental controls are situated.

The dinner table is set for eight. Miss Birch tells them that the Game of death is a survival game. Somewhere on the asteroid is a Nocturn – a merciless predator. The Doctor informs the other guests that a Nocturn can mimic exactly any other life form. It could even be in the room with them now. He says that a game like this is insane, they will be lucky if any of them survives. Miss Birch responds by telling them that is why the prize money is so high and she adds that the rule is the Nocturne can only hunt at night. The Doctor is just about to check if any of them is an alien imposter when the lights go out.

The Doctor grabs Gisella’s hand and drags her out of the room. He tells her that a Nocturne can actually hunt any time but is sensitive to light. He says that he doesn’t want to play this game; his chief problem is destroying the eternity Crystal. They hear a scream and a horrible tearing sound and run round corridors to the rear of the house where they find Lenny, dead. His blood has been drained. They step out onto the balcony where they find Scarlett. They are just about to scan her when another scream rings out and they run into the garden where General Korch has been killed.

They keep moving and find themselves in the space port. Horatio Hamilton is there, refuelling his ship. He tells them that he is leaving because he thought the game was a celebrity TV show and didn’t realise it would involve real killing. The Doctor accuses him of being a liar; he lied about how quickly he arrived and the Doctor doesn’t think he has any intention of really leaving. Then the spaceport power goes off, leaving them in darkness. The Nocturn slithers up to them but the Doctor uses the power hose nozzle to drive it off. He says the light from the end of the nozzle was enough to make it leave.

They leave Hamilton refuelling his ship, but only after he has promised to look after Gisella should anything happen to the Doctor. They go back to the house and into the cellar. The Doctor restores the lights to the house and grounds when Stokes arrives. They ask him to explain the climate controls and the Doctor experiments by pressing switches. Stokes tells him not to as they are the responsibility of Ted but the Doctor says they are security coded so his tampering will have no effect. He tries to use his screwdriver but again he says there is no effect. He explains that he needs Ted to switch the sun back on. A scream rings out and when they emerge from the cellar onto the gravel path they find Ted’s body. The darkness will never be lifted.

They run to Miss Birch in the house to ask for the security code but she says that it is the Game of Death and nothing can be done. She tells Gisella to run while she can and then transforms into the Nocturn. The Doctor pushes Gisella out of the door and turns to face the monster. He kicks the creature aside, taking it by surprise, then dives through the window.

Gisella runs towards the space port. On the way she is joined by Scarlett. Gisella begs Hamilton not to leave and he says that he wasn’t planning on it – there is more to him than he at first appears. Scarlet says the same is true of her and transforms into another Nocturn. Hamilton produces a wallet that shows him to be Karl Zalenby, Galactic Police. He says that the Game is a fix; the police have been following it for months, suspecting that a group of Nocturns have been using it as a front for their murderous behaviour. He produces a gun but the Nocturn’s tentacles lash out and he is slammed into the side of his ship, knocked unconscious.

The Doctor runs down the garden and then doubles back to the house. He enters the kitchen where he meets Stokes again. Stokes tells the Doctor to follow him down a secret route to the library. On the way, Stokes says that the Doctor isn’t like the usual guests and neither is Gisella – she isn’t human. The Doctor agrees, saying that Gisella is an android. Stokes asks if that means she is like the robot that recently arrived in the grounds. The Doctor realises with horror that he has been fooled by the Agent into coming to the asteroid so that it could get the Crystal.

The Nocturn is about to feed on Hamilton when Gisella drags it away. The creature and the android are about to fight when the Darksmith agent suddenly looms over them.

The Doctor runs through the house on his way to get to the spaceport, desperate to reach Gisella before the Agent can find her. On his way through the library he finds Professor Doofus. The pair of them, with Stokes close behind; go out into the garden where Miss Birch is waiting. She transforms into a Nocturn again. To the Doctor’s horror, Stokes and the professor rip open to become Nocturns, too.

The Agent and the Nocturn notice each other. Ignoring Harrison and Gisella they fight. The Agent gains the upper hand and rips the Nocturn’s head off. It turns round to find that the android and policeman have gone.

The Doctor holds up his screwdriver. He tells the three nightmare creatures that far from having him where they want him he has lured them into a trap. He lied earlier about the controls for the environment needing a security code and while he was in the cellar he set them to respond to his screwdriver. He points the sonic at the moon and switches the sun back on. The three aliens collapse instantly.

Gisella and Zalenby run through the sunlight while she tells him about the Agent’s mission to retrieve the Crystal. Behind them, the Agent starts shooting. Its energy bolts ricochet off the parked space ships drawing the Doctor’s attention. One blast hits a refuelling point and blows Zalenby off his feet, gashing his leg with debris. They hide in the shadows of a huge ship but the Agent finds them. As it steps into the shadows with them Gisella runs nimbly past it. Zalenby reaches up to a control panel and lowers the ship down on its legs. Too late, the Agent realises it is trapped. It tries to hold the ship up but is crushed beneath it.

The Doctor arrives and begins to help Zalenby back to his ship but the Agent manages to extract itself from under the craft that crushed it. When it stands it is deformed, stooping and cracked. It tries to follow them, using its one remaining eye to follow their progress. It utters threats but finds that its weapons are malfunctioning again. They turn to face it and the Agent throws the Doctor and Zalenby aside. Gisella takes on the robot in a furious fight. Just as the robot seems to have the upper hand the Doctor thrusts the refuelling nozzle into its innards. The Agent breaks apart and collapses, its green eye fading forever.