From Wikipedia

Rat'Lar, the maniacal leader of a species of aliens called the Scrubb, demands that Earth's greatest champion fight the greatest Scrubb fighter. If Earth refuses, the Scrubb and their huge armada of spaceships will destroy it. Superman and Muhammad Ali each come forward to volunteer. However, Ali argues that Superman is not really of Earth, and has an unfair advantage in his many superpowers. In typical Ali-style verbiage, he puts himself forward as the obvious choice.

Intrigued, Rat'Lar decides that Superman and Ali should fight one another to see who really is Earth's champion. To make the fight fair, he decrees that the match should take place on his home planet, Bodace, which orbits a red sun (which temporarily robs Superman of his powers). The winner would simply be the best boxer. The two would-be champions decide that Ali will train Superman in the finer points of boxing. They journey to Superman's Fortress of Solitude to have his powers temporarily deactivated.

The Superman vs. Muhammad Ali match is broadcast on intergalactic television to thousands of other worlds (with Superman's pal Jimmy Olsen acting as broadcaster). With the match underway, it soon becomes apparent that in battling with more or less equal strength, Ali is the superior fighter. Superman takes a serious pummeling, but somehow refuses to fall down; he stays on his feet all through the beating. Finally, Ali stops the fight, urging the referee to call for a technical knockout. Superman then falls face-first on the canvas (making the knockout more than technical).

Now crowned Earth's champion, Ali is set to face the Scrubb's champion, the behemoth Hun'Ya. The alien leader then asks Ali to predict at what round the fight will end. (Ali was known for predicting the round in which he would knock out his opponent.) After some chiding, Ali predicts that he'll knock the alien out in the fourth round ("He'll hit the floor in four!"). Once the match begins, however, Ali quickly starts to suffer from fighting the super-powered Hun'Ya.

Meanwhile, Superman's great recuperative powers have enabled him to make a speedy recovery. Disguising himself as Ali cornerman Bundini Brown, he steals into the Scrubb command ship and sabotages their space armada. In his showdown with the armada, however, Superman is again badly hurt, and is left drifting in space.

Miraculously, Ali gets a second wind. In the predicted fourth round, he not only knocks the alien champion out, but out of the ring as well. Yet after witnessing Superman's decimation of his forces, the Scrubb leader cries foul and decides to invade the now helpless Earth anyway. Just as Rat'Lar is about to give the go-ahead to his backup forces, his own champion Hun'Ya becomes enraged at Rat'Lar's dishonorable tactics and deposes him. There will be no invasion. Earth is saved.

Superman is rescued and once again revived. Hun-ya, the new Scrubb leader, makes peace with Ali, Superman, and all of Earth. The very end of the book shows Ali and Superman in a private moment. Ali reveals that he figured out Superman's secret identity as Clark Kent, but implicitly vows to keep it secret. The book ends with the two champions embracing and Ali proclaiming, "Superman, WE are the greatest!"