The X-Men (Marvel Girl, Cyclops, Iceman and Angel) and Havok and Lorna Dane are sent to an island to find a mutant, but all of them (save Cyclops) disappear. So, Cyclops told Professor X what happened. Prof. X gathered Storm, Nightcrawler, Bashee, Sunfire, Wolverine, Colossus and Thunderbird to save the X-Men. The team discover that the mutant that the first group looked for wasn't on the island. He is the island and feeds on mutants. So, the old and new X-Men gang up him and launch him into space.
The mutants Xavier gather would later become the second generation of X-Men.
The Essential X-Men