Stephen�s classmate, Robert, paints an accurate picture of Rexal 4, an alien world. Recognising it from his travels, Stephen tells John and Tim, and mentions that he thinks his teacher, Elizabeth, is breaking-out. The following day, Elizabeth witnesses Stephen in the act of jaunting home and hears him telepathically talking to John and Tim, but is convinced that it is a trick. The next day Robert's picture 4 has changed from a bright, sunny landscape to a dark and stormy picture. Investigating back at the Lab, John and Tim find similar instances of pictures that change, causing serious disturbances amongst schoolchildren. Stephen witnesses aggressive behaviour amongst his schoolmates. He takes John to meet Elizabeth, but she refuses to believe him and tries to leave, but as John jaunts in front of her to stop her, it triggers her breaking-out. She tries to run away again, but Stephen jaunts right in front of her. She panics and jaunts away, out of control.

John and Stephen jaunt into hyperspace to look for Elizabeth. When they find her she jaunts back to the classroom. Stephen and John manage to calm her down, and take her to the Lab. The next day sees Robert hand out blue and green badges; Elizabeth takes a blue one, while Stephen takes green. Chris arrives at the Lab after a fight with his brother Ginge; he tells John that the fight was due to Chris being is a Green and Ginge a Blue. John removes Chris's badge, returning him to normal. Back at school a fight breaks out in the school classroom, with even Stephen and Elizabeth taking part. That night John and Stephen return to the classroom and set up a video camera to try to record the picture changing. Meanwhile, Elizabeth passes a junk shop and sees another of the strange pictures in the window. As the picture changes, she is threatened by a group of boys.

John and Stephen hear Elizabeth's cries and jaunt to help, scaring the boys away. Stephen returns to the school and finds the video camera smashed, while John buys the painting from the junk shop, and discovers that Robert lives above the shop. Stephen is invited home by Robert, who shows him a cellar full of strange pictures, and boxes of blue and green badges; he tells Stephen that he did not know his parents, and that his grandfather is not really his grandfather. He also gives Stephen another picture. Meanwhile, John and Tim attempt a Neutron examination on their painting, but it destroys itself. Tim discovers that Robert does not appear in official records. Robert has been using the picture he gave to Stephen to spy on the Tomorrow People in the Lab, and when John returns to the shop, he is captured by Robert�s grandfather. Under Robert�s control, John telekinetically damages the Lab, then disappears.

John returns to the cellar, where his powers are negated. Stephen attempts to rescue John, but his stun gun has no effect on Robert, and he is forced to flee. Chris and two friends also mount an unsuccessful attempt. Chris later follows the mysterious boy, but is warned off by a policeman. However he manages to retrieve some books, including volume four of �The Decline and Fall of the Roman Empire�, which Robert has dropped. Taking them to the Lab, Chris shows Tim, who realises that the Roman Empire also broke into two factions: blue and green. Robert instructs his grandfather to tell the police that he has gone missing, then leaves with John. Attempting to help Elizabeth at the school, Chris is instead caught by the police, who charge him with Robert's disappearance, and take him to the shop to look for the boy. When they get to the cellar, all of the pictures have gone, as has John.

But Chris escapes, locking the policemen and Robert's grandfather in the cellar. Robert reveals to John that he is an alien Denagelee, but was born on Earth from an egg. When hatching, his species require aggressive energy to complete the transformation; the destruction of the Roman Empire was caused when his mother hatched - this time there will be many thousand of his race hatching. John persuades Robert to release him, so that the Tomorrow People can try to find a safe way for the aliens to hatch. As the hatching begins, Stephen suggests that if they could use a giant stun gun in space to put everyone to sleep - their REM state could provide the energy needed by the Denagelee, without hurting anyone. John, Stephen and Elizabeth collect the parts needed for the gun, and take them to the Watchdog satellite to assemble. Once activated, the stun gun sends everyone to sleep, allowing them to dream the violence the Denagelee require. The aliens leave Earth, agreeing never to return. John,

John and Stephen both share strange dreams of Peter, the Time Guardian, running through a forest, and showing them a vase. The two of them look through thousands of pictures of vases, eventually finding Peter�s, which is owned by Professor Cawston, and looked after by Professor Garner. Stephen tricks Professor Gamer into bringing the vase to him. He grabs its lid, and jaunts back to the Lab with it. The lid has strange markings which are designs for a Neutron Interton. The Tomorrow People build the device: a small disc. Nothing happens when Chris and Elizabeth pick it up, but when Stephen picks it up he disappears.

Stephen is caught in a time trap. While John attempts to make another Time Disc, Stephen meets a Guardian of Time called Zenon, Peter's grandfather. He frees Stephen and warns him not to build another Time Disc, as its user could be lost in eternity; Peter is trapped in Roman times, and Zenon gives Stephen five Time Discs so that the Tomorrow People can try to rescue him. Stephen arrives in the Lab just in time to stop John using his new Time Disc. Later, Professor Garner tells them that the vase came from a gladiator school in Silchester, run by a ruthless man named Gaius. John, Stephen, Elizabeth and Chris use the Discs return to ancient Roman Britain. They �sell� Stephen to the gladiator school, but then discover that their telepathic abilities have disappeared. Stephen is locked in a dungeon with Peter, who thinks that someone is transmitting a telepathy-inhibiting radiation. After Gaius selects Stephen to be tested by fighting, John, Elizabeth and Chris try to buy him back, but Ga

Gaius spares Stephen's life. Stephen meets Cotus, another trainee who is due to be killed in horse trials. That night John, Elizabeth and Chris break into the gladiator school, where they hear a steam engine, sixteen-hundred years before it should have been invented. Peter suspects that Gaius is responsible for the time warp. Chris manages to free Stephen, Peter and Cotus, and Peter attempts to arrest Gaius, but he escapes in a Time Pump. The trainee gladiators are released just before the Time Pump explodes, and the Tomorrow People return to the Twentieth Century. But the Lab is not there, instead, they are in a futuristic base populated by ape-like creatures. Cotus returns to the gladiator school and begins to repair the steam engine.

John, Stephen, Elizabeth and Chris discover that the Roman Empire never ended. The Romans developed advanced technology, which enabled them to rule the world, aided by their ape-servants from the planet Trista. John, Stephen, Elizabeth and Chris return to ancient Britain, but their conversation has been overheard by Gaius, who follows them to the gladiator school. They destroy the remains of the steam engine, but they and Cotus are all captured by Gaius. Cotus tries to stop them from being killed, and is knocked-out by one of Gaius' guards. Cotus loses his memory, and Gaius and his guards all disappear. Time has been restored to its correct path.

Stephen goes under-cover to investigate a military organisation called the "Doomsday Men". The leader�s grandson attends Glen College, which Stephen discovers is a training ground for Doomsday Men, and that its head, General McLelland, is planning a suicide mission. After Stephen is asked to join the organisation by Douglas, he discovers that the mission, "Operation Silver Thread", is to prevent the signing of a peace treaty. Stephen is summoned to the gym, where he has to fight several boys with a sabre.

Stephen, who was a schoolboy fencing champion, is able to disarm the boys. He is told that Operation Silver Thread is to happen the next day, and is to involve the Damocles space-station. One of the crew of the Damocles, Lee Wan, has an accident whilst working outside the space-station, and is blasted into space. John, Stephen and Elizabeth use a matter transporter to take Chris's transit van into space. They rescue Lee Wan, and take him back to the Lab. Stephen returns to the school, and that night is initiated into the Doomsday Men. Operation Silver Thread begins: General McLelland and his troops arrive at the Damocles in a space ship. Pretending to be the relief crew, they seize control of the station.

John, Stephen and Elizabeth decide to try to re-capture the Damocles. Lee Wan tells them of a dead man's switch that can launch nuclear missiles held aboard the station. John takes Lee Wan's space suit, and floats past the Damocles, pretending to be the dead astronaut. Looking through an observation port, he sees that the dead man�s switch is in use. The Doomsday Men transmit their demands to the United Nations, who postpone the signing of the peace treaty. Meanwhile, Stephen and the boys at the school go on a cross-country run. While Tim matches the voice from the Damocles with that of General McLelland, Stephen sees Douglas being kidnapped. He tries to help, but is knocked unconscious.

Stephen is found by his classmates, and taken back to the school. Douglas has willingly gone with the kidnappers to an old deserted windmill, and when one of the teachers, Dr. Laird takes some spare clothes to him, Stephen and Paul, one of the schoolboys, hide in the back of his car. But Dr. Laird captures them, and is about to shoot them when Stephen activates his jaunting belt, taking Paul and Douglas back to the Lab with him. Tim re-educates the two boys, showing them how terrible war actually is, and they decide to help. Dressed in Lee Wan's space suit, John drifts up to the space-station, and sees General McLelland at the dead man's switch. Douglas, Stephen and Lee Wan jaunt into the space-station, but in the confrontation, the General lets go of the handle. They are unable to stop the commencement of the launch countdown, but Douglas throws his bagpipes into the artificial gravity machinery that makes the living quarters rotate, causing the whole station to spin. The missiles fir