Xavier and his students make a graduation photography of themselves with diplomas in their hands. The Professor tells his students that they have to continue to protect humanity against evil mutants while he will be gone to deal with some unfinished tasks. In his absence he will name one of the X-Men as leader.
At another mansion across the town the Brotherhood assembles. Mastermind and the Scarlet Witch are the first to arrive and the illusionist offers Wanda a life full of luxury, but she refuses knowing that it is only fake; even if it were real she couldn’t stand being with Mastermind. As the rest of the Brotherhood arrives Magneto puts Mastermind back into his rightful place again.
Back at Xavier’s school the professor asks Scott to accompany him into a section that so far had been off-limits to the students. There he shows and explains to him an apparatus called Cerebro. It’s purpose is to detect mutant brain wave activity. Scott wonders why Xavier, a telepath, needs that machine, but it is intended to be used by the one who will replace Xavier as leader – namely himself. Cyclops is surprised that Xavier chooses him, he thought that the Angel is more aggressive or the Beast more intelligent, but Xavier points out that Scott‘s quality is leadership. The next morning Xavier has already left and the other X-Men congratulate Cyclops to the promotion. They plan to spend a day in the city and ask Scott to accompany him, but he refuses as he has to monitor Cerebro.
At a carnival place Magneto walks around in costume without arousing any curiousity. He is intrested in the performance of one artist called the Blob and after he saw that the fat man could not be moved by an elephant or a cannon fired at him, Magneto is convinced that he found another mutant. Trying to probe him, Magneto discovers a mental block.
Suddenly the Blob’s manager appears, he doesn’t want his client to talk to other people. Magneto calls the Brotherhood who fight the carnival people till all of them run away frightened.
Now alone with the Blob Magneto orders him to join the Brotherhood, but he doesn’t want to and attacks the Master of Magnetism. During the fight the Blob overcomes his memory block and wanting revenge on the X-Men he agrees to join the Brotherhood.
Right this instant Cerebro’s alarms go off and Cyclops sees that Blob’s name is highlighted. Quickly he checks the sign-out book about the X-Men’s current whereabouts and tries to contact the Angel through his car radio.
Hank and Bobby are enjoying the music in a Greenwhich coffee shop, but as his shoes are to tight the Beast takes them off for a moment and becomes the star of the evening. Nobody ever saw such large feet and the party people celebrate him and want to start a new barefooted cult. While the others are busy Bobby starts to get involved with the waitress Zelda.
Warren comes into the shop and alerts his teammates to the danger of the Blob; they all leave to meet with Cyclops and Marvel Girl.
Magneto challenges the X-Men to fight the Brothrhood at an abandoned factory and as they arrive a fight starts between the two teams. Magneto even fires some missiles on his opponents, but Cyclops proves his leadership qualities as he barks the right orders and his team is able to deal with both the villains and the missiles.
Suddenly as the X-Men stand in one group Magneto fires a second set of missiles even though the Blob is in the way. The X-Men hide behind the fat mutant who takes the full impact of the explosion and is only knocked down to his feet. Seeing that his plans won’t work, Magneto orders the Brotherhood to retreat.
After they are gone the X-Men offer to help the Blob since he was abandoned by his pals, but he refuses; he’s fed up with fighting other people's fights and only wants to return to his life at the carnival.