A Button in the Fabric of Time

After thousands of years of atomic warfare, the Antons on planet Ergo had mutated so badly that they were no longer recognizable as human. They escaped to planet Duos in Galaxy 3 and established a new civilization. They wanted to restore their bodies, so they searched the universe and found people living on Earth in the thirty-first century with perfect bodies. The Antons wanted to use the earthlings' DNA, but the earthlings had powerful weapons and wouldn't allow the Antons to enter their atmosphere.
The Antons place a time-travel device in the hands of Gus Wilder, an American engineer of the twenty-first century, hoping he will serve as their emissary in dealing with earthlings of the thirty-first century. They influence Gus to travel a thousand years into the future where he finds an advanced society who have created a paradise on earth. These earthlings have overcome every human frailty and provided for every need.
The women are all beautiful and the men are all handsome. These earthlings live long into the future and strive for immortality. Gus meets and falls in love with an earth woman called Jan-3. The Antons tell Gus why he was chosen and, if he fails, they will destroy planet Earth. He and Jan-3 set out to save humanity. Can they fulfill their destiny?