Scott walks into the Danger Room to find Marvel Girl, Iceman and Beast practicing their powers except for Angel. They find Warren in his room watching the news reports of a man “clad only in a loin cloth”, accompanied by a Sabretooth Tiger, who saved an expeditioner in Antarctica.
Shortly after, Xavier calls the X-Men into his study. He is certain that the man on the news isn’t a mutant, but nonetheless he has allowed the bored X-Men to go on a mission to investigate with Cyclops leading the team.
In Antarctica, the X-Men discover a hidden tunnel leading into warm caverns. Upon exiting the tunnel, they discover a land with roaming dinosaurs. They are attacked by “primitive warriors”. The man on the news with the Sabretooth Tiger appear to scare the warriors away, but not before they retreat with Marvel Girl. The man introduces himself as Ka-Zar, and his Sabretooth companion is named Zabu. Ka-Zar agrees to help the X-Men find their captured team mate.
While Angel is off scouting in the air, he too is captured by the warriors. He and Marvel Girl are to be sacrificed to a T-Rex. Marvel Girl uses her telekinesis to topple the dinosaur. Then she unties Angel and they attempt to escape by flying off. Meanwhile, the Ka-Zar and the rest of the X-Men try to break into the primitive fortress. Ka-Zar summons the help of a stampede of Mastodons to break down the fortress.
Marvel Girl and Angel are rescued from the warriors. The X-Men bid Ka-Zar farewell and return home.
(X-Men: The Early Years #10 is a reprint of Uncanny X-Men (Vol 1) #10.)