Batman and Robin are scooped in their attempt to capture some thieves at an airport by Mr. Marvel, a new costumed crime-fighter, who uses futuristic gadgets and rides in a sky-car. In a press conference, Mr. Marvel says that he is a scientist and that he believes Batman's crime-fighting methods are outdated, as lawmen should not have to use fisticuffs.
As Mr. Marvel continues to make headlines, Bruce Wayne notices Dick Grayson seeming to hero-worship the newcomer. Finally, Robin packs up and announces he is leaving to become Mr. Marvel's partner. Batman tries persuasion, which fails, and then tells Robin that can stop him from leaving as his legal guardian. Robin replies that he would have to reveal his Bruce Wayne identity in court to do so, and that, after a trial period, he will have his features changed by Mr. Marvel to start a new identity. The two heroes make a strained farewell.
Later, Robin mulls over the real reason he has joined Mr. Marvel: to save Batman's life. Earlier, Marvel threatened to kill Batman if Robin did not become his partner, demonstrated his power by using a disintegrator ray-gun on a Batman billboard, and indicated that a device on his belt was tuned into "Batman's frequency" and would send a death-bolt to Batman if Robin did not team up with Marvel. Robin and Marvel continue their crime-busting, but Batman, recovering partially from the shock, suspects that Robin has not given him the true reason for his departure, and begins shadowing the pair.
During another encounter with crooks, Robin breaks off the fight, slugs Mr. Marvel, and tears away his belt. Batman joins in the fracas and helps Robin put down the brigands, and afterwards, learns the truth from Robin. Batman braces Marvel and unmasks him, revealing Marvel as an alien. The alien confesses that he and his partner had long observed Batman and Robin from their hovering spacecraft, and that Marvel had made a wager he could acquire Robin as a partner for ten days. He confesses that his belt had no power to tune in on Batman's frequency, makes a lame apology, and is whisked back into his ship by his partner. Batman muses that Robin was only his partner for nine days, so Marvel has lost his bet. The two heroes head for home, reunited again.