Without warning, the Decepticons attack Stansham Castle in southern England, dropping a bomb that is buried deep into the ground. While castle curator Roy Harker studies the situation, his son Sammy accidently encounters Jazz in the nearby woods. Sammy runs home scared and is secretly tailed.
That evening, Roy tells Sammy an old story from Stansham Castle, about a giant "man of iron" that defended the castle when it was under siege. When Roy shows Sammy an illustration from an ancient manuscript, Sammy confirms that it looks like the robot he saw earlier, which leaves them to wonder if the Man of Iron has reappeared.
Later that night, Sammy dreams of flying spaceships and giant robots when an invisible force shakes him awake. The drawing of he Man of Iron drifts out the window, and Sammy's cries wake up his parents, who arrive just in time to see Mirage returning to the nearby woods.
The next morning, British military informs Roy Harker that a sonic scan reveals a mysterious, massive obect beneath the castle -- something the size of an ocean liner. Meanwhile, Sammy is walking down the street when he sees a parked Porsche near his house -- with the Man of Iron picture inside. Jazz talks Sammy into climbing in for a chat. Just then, Sammy's mother comes out to see what's going on, whereupon Jazz drives off with the boy.
Jazz is taking Sammy to an Autobot shuttlecraft when they are attacked by Decepticon jets. With the help of Mirage, Trailbreaker, and Bluestreak, they evade the Decepticons and reach the shuttle. Once there, they are joined by Optimus Prime, and the secret of Stansham Castle is revealed: the massive object buried underneath it is an Autobot rescue ship, sent to Earth from Cybertron. The Decepticons are also investigating the signal, and the Autobots must beat them to it; if the Decepticons can find the ship first, they will destroy it -- and the humans living nearby -- to keep the Autobots stranded on Earth.
At the castle, the British military have started to unearth the ship. They know that it is releasing a signal of some sort, and suspect that it is an extraterrestrial spaceship. Then, while Roy Harker muses over this newest twist of affairs, the earth shakes -- and the Man of Iron rises out of the ground. He is starting to attack the human's vehicles, only to be destroyed by Starscream.
The Autobots are racing to the castle via their shuttlecraft when the Decepticons attack. Jazz drops Sammy off to safety and attacks Starscream, while the shuttle fights off the other Decepticons and sends them fleeing.
After the battle, Optimus Prime concludes that the Decepticon danger is too great for the Autobots to leave Earth, and orders Jazz to destroy the buried rescue ship -- and its hibernating Autobot pilot. After the Autobots depart, the British government removes the debris and supresses the story. The ruins of Stansham Castle eventually gain notoriety as a foci point for UFOs and supernatural events, while Sammy Harker is left with fading memories of a Man of Iron...