Passing through the 25th Century, Superman bears witness to Professor Zoom murdering the Flash. Professor Zoom runs around the Flash at super speed, both clockwise and counter-clockwise, creating a vibratory frequency that threatens to rend the Flash to pieces, molecule by molecule. A quick puff of Superman's super cold breath revives the Flash. The Scarlet Speedster vibrates his body underground, burrowing out from under Professor Zoom's circular path. The Flash then strikes Professor Zoom from behind, laying him out on the pavement. The Flash speeds off, unaware that Superman is nearby. The Man of Steel builds a makeshift prison for Professor Zoom, dislodging blocks of pavement from the plaza for building materials. Professor Zoom takes off in pursuit of the Flash, little suspecting his nemesis has anticipated his attack. The Flash darts behind a corner. As Professor Zoom speeds towards him, the Flash extends his leg. Moving too fast to stop, Professor Zoom trips over the Flash's leg, and barrel rolls into the pavement. The Flash places his old foe into the prison cell built by Superman, then continues moving forward, into the future. Superman follows Flash into the time stream. Both heroes' progress is monitored by the commanders of two alien ships. Superman catches up with the Flash. As the Flash has been drafted into assisting the Zelkot, Superman has been pressed into service by their mortal enemies, the Volkir. The Flash is stunned that Superman would consent to opposing him, on behalf of the Volkir, as the Flash's mission for the Zelkot is being done to keep them from destroying the Earth. Superman explains that if he doesn't stop the Zelkot's agent, Iylar, from breaching the time barrier between the future and the past, to prevent the civil war between the Volkir and the Zelkot from ever starting, Superman will cease to exist.
If the two races don't separate over the conflict, and set off for the stars, then the Volkir will not journey to Krypton, and stabilize it's core, thousands of years before it's inevitable destruction. Without the Volkir's stabilizing influence over Krypton, Superman's home world will have exploded prematurely, thus erasing Superman from existence. The Flash is aghast that Superman would be willing to let the Earth die just to save his own skin. The Flash and Superman close in on Iylar, just as the Zelkot revolutionary impacts with a Volkir temporal mine. The Flash is thrown into the 27th Century. Superman flies ahead of Iylar, and clears the time stream of the Volkir mines. Then he turns on Iylar, and beats the alien into unconsciousness. Iylar's time scooter continues to carry the alien forward in time, despite his lack of consciousness. Superman backtracks to the 27th Century, and repairs the rend in the fabric of space/time caused by the temporal mine's detonation. Recovered, the Flash re-enters the time stream and catches up with the unconscious Iylar. Abruptly they both come to a stop. Superman also re-enters the time stream, but is suddenly shunted back out, and finds that he cannot leave the 30th Century. The Flash and Iylar find themselves inside a metal cylinder. While the Flash works on getting out, he notices that Superman has left a message etched into Iylar's armor, asking for the Flash's trust. The Flash uses his vibratory powers to shatter the metal cylinder. Once outside of it, Iylar's time scooter automatically carries him back into the time stream. The Flash, however, finds himself under attack by spear wielding turtle men. Superman travels to the headquarters of the Legion Of Super-Heroes. Encountering his younger self, Superboy, the Man of Steel ascertains what the problem is, and determines a solution.
The Flash speeds back into the time stream, but before he vanishes, a stray spear strikes his wrist, destroying the Zelkot's monitoring device. Superman and Superboy fly at top speed into one another. The incredible impact of an irresistible force crashing into an immovable object hurls them both back to their respective eras. Superboy lands in Smallville, in the 1950's, while Superman is thrown back to Metropolis, in 1978. Superman jumps back into the time stream, and quickly catches back up to the Flash. The Volkir monitoring device on Superman's wrist was destroyed on impact with Superboy. Finally able to speak freely, Superman explains that if the war between the Zelkot and the Volkir doesn't start, then the Zelkot will have never come to Earth. It was their presence that lead to life appearing on Earth. The civl war between the Zelkot and the Volkir must happen, or else all life on Earth, as well as the life that once inhabited Krypton, prior to it's destruction, will be forfeit. Iylar reaches the barrier between the future and the past, and passes through it. The Flash vibrates his molecules into intangibility, allowing Superman to pass through him, as they both penetrate the barrier simultaneously, behind Iylar. The Flash beats down Iylar, while Superman destroys Iylar's time scooter. The two heroes continue moving forward in time, through the past, until they reach the present. Carrying the Flash into Earth's orbit, Superman uses his super-cold breath to take out the alien ships' life support systems. Gaining entry through an airlock, Superman takes out the remaining aliens while the Flash insures that the Earth is no longer imperiled. Superman hurls the two alien vessels to the other side of the galaxy. The crisis passed, Superman returns to Central City, with the Flash, for a cup of coffee.