Challengers of the Unknown comes to an end in Super-Team Family 10 (April/May 1977), which is also the last issue to contains a reprint, another great Doom Patrol one.
Skeates, Sherman and Abel open the issue with a nuclear test, and Multi-Man, the old enemy of the Challengers, shows up to allow it to kill him, so he returns with new powers, quite potent and deadly ones.
The Challengers had believed that Multi-Man was still imprisoned on their island base, so they head out their to check on him and the other prisoners, Kra and Volcano Man. All three bad guys had last been seen in 1968, and this brief cameo is the last appearance for both Kra and Volcano Man. So they must never get off that island. Multi-Man, however, constructed a dummy to take his place, and Rocky Davis, Red Ryan, Professor Haley and Ace Morgan fire around accusations of blame as to who should have noticed this, not that it much matters now.
Multi-Man announces his plans to destroy the various cities of Earth, using his new invulnerability to survive the destruction, unless the world allows him to become dictator of it. The Challengers note that he has gone on a crime spree first, and by looking at what he has stolen already, deduce what he might be going for next, to create his bombs. At the same time, Professor Haley starts working on a formula to neutralize his powers. June Robbins shows up with Gaylord Clayburne, a millionaire who once tried to join the team, back in 1963. At the end of that story he and the Challengers had become friends, but in this tale they are openly hostile towards him. Also, his name is now spelled slightly differently. It was Clayburn in the original tale.
The team then intercept Multi-Man at the site of his next robbery. They discover that his invulnerability is really intangibility, and Haley's formula proves useless, going right through the villain.
Clayburne shows up with a weapon he invented to stop Multi-Man, but by then the villain has taken off, bringing June along with him.
The finale sees Multi-Man threaten to kill June if the Challengers continue to try to stop him. Clayburne is furious with them, acting as though June is now his girlfriend.
The story continues in the revival of the Challengers of the Unknown series, which comes out a couple of months after this story.