Professor Zoom returns after fooling prison officials and passing an electronic psychological examination, ensuring his release.
After a year in prison, Eobard Thawne, the notorious Professor Zoom, is released. A psychic scan has indicated that Thawne is completely reformed. In truth, Thawne tampered with the test to get the desired results. While in prison, Thawne envisioned an entirely new branch of science, one dedicated to the study of Ultra-Speed. The first device Thawne creates, using the principles of Ultra-Speed, is a machine that causes the evil side of one's nature to become dominant. Having successfully tested the machine on laboratory animals, Thawne sets his sights on Al Desmond. A brilliant chemist, Desmond was once the notorious Mister Element.
Having served his time, Desmond tried to put his past behind him. Thawne tried, once before, to set Demond back down the path towards evil. Now, Thawne intends to try again. Thawne projects the machine's radiation, 500 years into the past. Desmond is in his supervisor's office, about to accept a promotion, when he feels the effects of the radiation. Immediately, he calls his friend, police forensics scientist, Barry Allen, and asks him to dinner. Desmond begs Allen to contact the Flash. Allen exits the restaurant, then returns, as the Flash. Desmond tries to rob the restaurant, but the Flash hauls him out before the crime can really get started.
The Flash offers to aid Desmond, but Desmond turns on him. Taking the Flash by surprise, Desmond punches the Flash unconscious. Desmond returns to his old criminal lair, and puts on his Mister Element costume. All it once, Desmond shakes off the irresistible criminal urges that overcame him. Thawne is unable to keep the machine's radiation focused on Desmond. As it passes through time, the radiation begins to spread out. Suddenly, Central City is awash in criminal activity. The Flash rounds up the would-be criminals, handing them off to the police. Unable to narrow the radiation beam to just Desmond, Thawne travels back in time, himself, along with his Ultra Speed machines.
Using a Cosmic Treadmill of his own design, Thawne returns to the Central City of 1965, as Professor Zoom. Using his ability to dominate the minds of men, Professor Zoom forces Central City's Municipal Council to outlaw super-speed travel within city limits. So steadfast is his belief in the law, the Flash refuses to run at super-speed, despite Professor Zoom's taunting. Professor Zoom even goes so far as to rob a bank right before the Flash's eyes, but the Flash makes no super-speed move to stop him. Vibrating his molecules into intangibility, the Flash slips below the streets of Central City.
Professor Zoom tracks the Flash down, just as he exits a tunnel, leading up from Central City's underground. Suddenly, the Flash lunges at Professor Zoom, once more using his super-speed powers. The Flash directs Professor Zooms's eyes over to the city limits sign. Professor Zoom, spinning at high velocity, breaks free of the Flash's grip, and races back across the city line. The Flash builds up a battering ram of compressed air, that floors Professor Zoom. The Reverse Flash recovers, though, too quickly for the Flash to take advantage of the situation. Desmond watches their battle, desperate to help the Flash, but still wrestling with Professor Zoom's evil impulses.
Professor Zoom defeats the Flash, and renders him helpless with ultra-radiation. From a monitor screen in his hidden lair, Professor Zoom forces the Flash to watch Desmond, as Mister Element, resume his criminal career. Professor Zoom leads Mister Element to his secret lair, then offers him the opportunity to murder the Flash. Mister Element takes aim, then fires at Professor Zoom's Ultra-Speed machine, freeing the Flash. Before Professor Zoom can react, the Flash knocks him unconscious. No longer consciously able to maintain his internal vibrations, Professor Zoom is drawn back to his own time. The Flash turns to thank Mister Element, only to have his former foe turn on him.
Prior to succumbing to Professor Zoom's evil impulses, Desmond had planted a post-hypnotic suggestion upon himself to aid the Flash on sight. Having done so, Desmond, as Mister Element, was now free to murder the Flash himself. Moving his arms at super-speed, the Flash blocks Mister Element's attack, while also knocking him unconscious. Using his own Cosmic Treadmill, the Flash carries Mister Element 500 years into the future. After learning that Professor Zoom has already been incarcerated, the Flash asks the scientists of the day to use their technology to reform Desmond. Cured of his criminal inclinations at last, Desmond accepts his promotion at work, and moves forward with his marriage plans, with his fiancee, Rita.