- Marvel Girl
- Dr. Kurt Marko - (Appearance in flashback only), (First appearance), (Death)
- Sharon Xavier-Marko - (Appearance in flashback only), (First appearance),
Cerebro detects the presence of an incredibly powerful being outside of the mansion, so the X-Men activate the mansion's defensive measures. The villain is able to break through line after line of defenses, so Professor Xavier informs his students of who the villain is so that they know what they will be fighting. Professor Xavier informs them that it is Cain Marko, his own stepbrother. Professor Xavier tells his X-Men the origin of their two families. After Professor Xavier's father was killed in an accident, Dr. Kurt Marko began courting Charles' mother, and eventually the two married. Sharon dies of a broken heart shortly into the marriage. One day Charles overhears Cain and Dr. Marko talking about inheriting the family fortune. Charles interrupts and a fight breaks out. A fire starts but Dr. Marko is able to get both boys out of harms way, but he dies from smoke and fume inhalation. After his death, Charles realizes his powers exist and he uses them to his advantage. Cain gets jealous. At one point, Cain offers to take Charles to school and he runs the car off the road. Professor X cannot escape the car as it falls over a cliff. The X-Men ask if this is how he lost the use of his legs and he says no.Years later, Cain and Charles both joined the army and served together in Korea. After a fire fight, the two stepbrother stumbled upon the Temple of Cyttorak. Cain finds a ruby in the center of the cave that transforms him into a human juggernaut. A bomb causes the cave to crumble, but Charles is able to get out just in time. Back in the present, Juggernaut breaks down the doors to the mansion and easily defeats the X-Men. Nothing stands in his way of Professor Xavier anymore.
Written by: Stan Lee
Layout by: Jack Kirby
Pencilling: Alex Toth
Inked by: Vince Colletta
Lettered by: Sam Rosen
- Professor X's full name 'Charles Xavier' is revealed in this issue.
- The back-story of Professor X's flashbacks of Alamagordo, New Mexico in this story will be revisited in X-Men Vol 2 #12 & 13
- The name of the atomic test site city is misspelled as "Alamagordo" instead of the correct name "Alamogordo".