The X-Men are right in the middle of a training session as Xavier interrupts since his mind located another mutant presence. He orders the team to switch into street clothes and sends them to New York to search for the unknown mutant. As they leave Xavier worries about Jean. From his thoughts it is clear that he loves her, but he can never tell her, since he is confined to a wheelchair and as the X-Men’s mentor has greater responsibilities.
In New York the team splits up and after a few wrong tracks (man using a magnifying glass to burn paper, “floating“ worker on a glass shield, trick shooter) Cyclops finally discovers a fat man called the Blob in a carnival. Professor Xavier who stayed in mental contact with his students confirms the Blob as the presence he felt earlier.
During the carnival show the Blob demonstrates his powers : even though six people at once try to move him he doesn’t budge, also bullets fired at him do not penetrate his skin. Later, after the exhibition the X-Men approach the Blob, but his attitude is rather unfriendly; only after he becomes aware of the beautiful redhead he decides to follow to the mansion.
There Xavier runs some tests on the Blob and discoveres that he can control the molecules of his flesh and offers him to join the school, but the Blob refuses. Actually he laughs at the X-Men since he believes himself to be more powerful than all of them together. Professor Xavier realizes that he made a mistake, he simply took for granted that the new mutant would join the team; now the Blob knows the mansion’s location and the X-Men’s secret identities. That is why he decides to erase the Blob’s memory of all that has transpired at the mansion, but the Blob is not too happy about it. He starts to fight the team and is able to escape. Outside the mansion he enters the sewers and makes his way back to the carnival.
He knows that the X-Men won’t allow someone to know their secrets and expecting another fight, he decides to attack them first. With his new knowledge of being a mutant the Blob takes over the complete carnival and by promising to make them all famous the Blob turns the various artists and acrobats into an army.
From afar Xavier realizes the Blob’s intend and starts to design an intensifyer so that he can erase the memory of the whole crowd, but before he has chance to finish, they attack. Led by the Blob the acrobats, marksmen, artists and animals attack the school and the X-Men are outnumbered. One by one they are defeated and tied up.
The Blob and his men now search the insides of the mansion for Xavier himself and leave the team unguarded; a fatal mistake as Marvel Girl is able to pick-up a knive from a knive-thrower and cut the X-Men free. They head into the masion and arrive just in time to keep the Blob and his crowd busy long enough for Xavier to use his completed thought intensifyer. The memories of the Blob and his crew are erased and they return to work in the carnival as if nothing happened.