The Avengers are supervising a chess match, financed by Tony Stark, between grandmaster Sporadnik and computer Nimrod. Sporadnik collapses during the match due to a rare Amazonian poison on one of his pawns, and is taken to hospital. Thor follows, changing to Dr Don Blake to treat the patient. But even knowing the cause he can't bring Sporadnik out of a coma. He suggests only the man responsible may be able to solve the problem. Meanwhile Sporadnik's condition is causing an international incident.
That night Leonard Tippit emerges from a hiding place within the computer, to be met by Vision hiding wraith-like within other machinery. Vision had suspected Nimrod had been tampered with when it invited the pawn move with a suspect move of its own. Tippit starts to glow and they fight. Vision tries his trick of partially solidifying his arm within Tippit's chest, but it is Vision who collapses unconscious.
Tippit hadn't wanted to hurt the Vision, who had sensed no evil in him. He now holds back when faced with his boyhood hero Captain America, giving him time to simply knock Tippit out. Then the rest of the Avengers turn up, having also suspected the computer was involved. But the unconscious Tippit fades through the floor. And the Avengers are hurled outside of time and space.
Tippit is actually asleep in his bed. But in his sleep he remembers how the Watcher came to him 2 nights before to tell him that only he could prevent a nuclear war that would happen in all possible futures. But to do so he must kill 5 innocent people who would otherwise indirectly cause the war. To this end Watcher gives Tippit the power to become a being of pure mental energy.
The Avengers have also seen this exchange in a vision, and now return to normal spacetime. They also know the identities of Tippit's intended victims and decide to save them, because the death of innocents is too high a price to pay even to save the world.
There follows a typical split into teams. Thor goes to protect a young boy in Naples. Captain America and Hawkeye head for a Masai woman. Vision takes a young Japanese composer. Quicksilver and Scarlet Witch try to help an old woman growing roses. They all fail to stop Tippit zapping his victims. But it has all taken its toll on his power, and Wanda manages to make him revert to his powerless state.
They take his unconscious body back to where Iron Man has built a Mentality Retrogressor. This stops Tippit from rebuilding his power. Then the Watcher appears and tells them that Tippit himself was the unwitting danger to Earth's futures. The 5 innocents were just a ruse to get Tippit to expand his energy, and to get the Avengers involved to capture him. Now that he is depowered the Watcher can take him away without risking causing the threatened catastrophe. The 5 innocents will recover.
The Avengers now say that Tippit himself is an innocent, because he wasn't going to cause the future war deliberately. They can't let Watcher take him. But Tippit asks them to let him go, for the sake of the Earth. They promise to remember him as the man who saved the Earth.