Terence "Terry" Kildaire works as chief of control at the Research Establishment, an atomic research facility nebulously connected in some way to the Home Office, and he's in big trouble. He's on trial for double homicide. Evidently, he murdered his assistant controller Rawlinson and a departmental chief named Royston. It takes the jury just ten minutes to convict him. But due to the lack of a clear motive, Kildaire is remanded to the custody of Long Meadow, an asylum for the criminally insane. There, Dr. Len Williamson and his colleague Dr. Bill Coe discover a scar on Kildaire's head, despite there being no record him having had surgery.
Meanwhile, in a small town near the Research Establishment, "Ticky" George wanders into the Blue Dragon for his usual several glasses of shandy and armed with a weird story about something he witnessed in the woods that night. He claims he witnessed a big silver ship come down in the woods the other night and disgorge a bunch of "tin men." Something about his story rings true for the landlord, who presses George for more. In-between sips of shandy, George can only report that the metal men wandered around a bit, then got back in their ship and left.
Back at Long Meadow, Kildaire is lucid enough to explain he doesn't remember committing the murders, and in fact remembers nothing after witnessing a spaceship land in the woods one night. An examination of the scar on Kildaire's reveals the presence of some kind of implant in his head. A quick call to MI5 brings Colonel Henry Rothman running. Rothman, along with his aide Major Richard Bruce, decide to go visit the Establishment. They're particularly interested in the woods around the area where Kildaire claims he saw the ship land.
A Geiger counter wielded by Bruce confirms the earth to be mildly radioactive. Now they definitely know something is up. It is Colonel Rothman's theory that whoever or whatever landed in the forest that night put the weird device into Terry Kildaire's head and made him kill Rawlinson and Royston, most likely to disrupt Britain's atomic research.
Later that night, the ship returns. Town drunk Freddy O'Flannagan is on another bender. Caught wandering the streets late at night by his lonesome, he is doing his very best to convince Constable Dalrow that he is sober and failing when the two hear the approach of loud, clanking metal footsteps, as an army of silver robots appears through the fog with the conquest of Earth very much on their minds.
Tropes used in this novel:
- The Brigadier: Colonel Rothman.
- Disintegrator Ray: The robots' primary offensive weapon.
- Distressed Dude: The police superintendent gets captured and held prisoner by the robots after trying to make friends with them.
- Dr. Jerk: Dr. Watson Smith, the physician who tended to Rawlinson. He is extremely arrogant and considers even testifying for the prosecution at Kildaire's trial to be a waste of his precious time.
- Empty Piles of Clothing: Anyone zapped with the robots' Disintegrator Ray is reduced to just empty clothes and ashes.
- Flying Saucer: The robot ship is pretty much described this way.
- Five Rounds Rapid: This is Rothman and Bruce's initial plan to deal with the robots. It doesn't work.
- Frame-Up: The robots frame Kildaire for the murders of Rawlinson and Royston. At first, it seems that they brainwashed him into killing them, but it's later revealed they killed the two men themselves, then somehow got out of the room before Mrs. Bloggins and Blake came in, leaving Kildaire to take the fall.
- Genre Shift: It starts out as a Courtroom Drama with Kildaire's trial, then switches to a Medical Drama with him at Long Meadow and Williamson and Coe's discovery of the weird implant in his head, and then suddenly it shifts into Science Fiction with the arrival of Killer Robots in a Flying Saucer.
- Killer Robot: The robots of the title are pretty bog standard killer alien robots.
- Mars Needs Women: The cover plays up this trope, but nothing like it happens in the novel. The robots are just there to conquer humanity in general, and pay no special attention to women.
- Master Computer: It turns out that the robots are controlled by one of these aboard their ship. This results in them having a Hive Mind (or perhaps a Mind Hive might be a better description), which turns out to be a huge weakness. If the central computer brain is destroyed, all the robots automatically shut down like the battle droids in The Phantom Menace. And that's precisely what happens after Terry Kildaire chucks a bunch of Gelignite into their ship, blowing the crap out of it.
- No Body Left Behind: Anyone hit with the robots' ray guns.
- No Name Given: The landlord of the Blue Dragon and the police superintendent.
- Outrun the Fireball: After Kildaire throws some Gelignite into the robots' ship, it creates a massive explosion, which he and the superintendent have to outrun.
- Police Are Useless: Zig-zagged. While the superintendent and his men aren't stupid, their weapons are of course useless against the mechanical invaders.
- Science Hero: Len Williamson and Bill Coe. Williamson is described as resembling "a balding Viking," and Coe is a former professional boxer.
- Some Kind of Force Field: What the robots surround their ship with.
- You Have to Believe Me!: Generally averted, due to the fast pace of the story necessitating that the characters not be idiots. The landlord of the Blue Dragon believes George's story because he doesn't think "Ticky" George would come up with something so fantastical. Meanwhile, after being told by Freddy the drunk about Constable Dalrow's death, Sergeant Richardson immediately goes to where Freddy said it happened and finds what's left of his subordinate, rushing immediately back to the station to phone for help.