The powerful, criminal, secret orginization Factor Three sent agents Ogre and the mind-controlled Banshee after the X-Men in an attempt to kidnap Professor X. Freeing Banshee from Factor Three's control, the X-Men easily defeated Ogre and foiled the plot.
Factor Three struck again when the Professor attempted to cure his step-brother of being the Juggernaut. The resulting experiment of both science and sorcery ended with the Juggernaut's revival and the Professor near the brink of death. Factor Three mentally contacted the Juggernaut and tried to get him to join their group. The X-Men ultimately stopped the Juggernaut with the aid of Dr. Strange, helping force Marko into the Crimson Cosmos. However, during the process, Factor Three kidnapped the Professor.
When Banshee attempted to contact the X-Men after learning of Factor Three's European headquarters, his message was interrupted, leading to the X-Men briefly battling Spider-Man, mistakenly believing the Wall-Crawler was a member of Factor Three.
Arriving in Europe, the X-Men were quickly captured by Factor Three, whose membership included the Vanisher, Blob, Unus, Mastermind as well as two new villains: the Changeling and Mutant Master. The group found the X-Men "guilty" of betraying mankind and sentenced them to death. Before executing the X-Men, Factor Three revealed that they intended to initiate World War III by attacking both the United States and Russia, sparking a nuclear exchange between the two. The X-Men split into two, foiling both plots and confronted Mutant Master at his headquarters in the United States.
There, with the aid of the Changeling, they freed the Professor. Mutant Master was revealed as a member of the alien race, Siris. The X-Men and Factor Three worked together to stop Mutant Master and parted peacefully but vowed to meet again as enemies.
- Mutantmaster (leader)