John and Stephen find themselves with two mysteries to investigate - the possibility that Stephen's new teacher Elizabeth M'Bondo is a Tomorrow Person and some worryingly accurate paintings of alien planets.
John and Stephen follow Liz into hyperspace, where they manage to rescue her and return her to the lab. Convinced of their story, she agrees to help them investigate the paintings, which seem to induce violence in those around them.
John and Stephen rescue Liz from the youths and find out about the second painting. John buys it from a man who turns out to be Robert's grandfather and takes it back to the lab, only to find out it may be a trap.
Robert explains to John that his race uses violent energy as a power source. If they stay on Earth, they will die, but the violence needed to power their departure will complete destroy Earth's civilisation.
A new peace treaty is about to be signed but the Tomorrow People are worried about the Doomsday Men, an organisation dedicated to the perpetuation of war. Stephen is sent undercover at the same school as the grandson of the group's leader.
Stephen earns the respect of Douglas and the others and is invited to join the Doomsday Men. Meanwhile, an accident at the Damocles space station leaves an astronaut in danger, so the Tomorrow People decide to intervene.
The Damocles is in the hands of the Doomsday Men and the Tomorrow People are powerless to act. Any attempt at intervention and McLelland will throw the dead man's switch and launch nuclear missiles towards the Earth.
Stephen and Paul suspect Laird is involved in Douglas' kidnapping and follow him, only to learn the whole incident was set up by the Doomsday Men. Stephen teleports both boys back to the lab where the Tomorrow People try to convince them to help stop McLelland.