When Captain America invites the X-Men and Magneto to join forces with the other Marvel heroes. The X-Men had spent the first half of the series working separately from the others. After teaming up with Magneto (who was accidentally grouped with the heroes but went off to do his own thing as well), they rescued the battered heroes from an ambush by the villains. After the bad guys escaped, Cap shook hands with Cyclops and formally invited the mutants to fight alongside them. Standing behind Cyclops are Storm (in her mohawk phase) and Nightcrawler, driving home the notion of outcasts being accepted by the conventional heroes. They fight as a single unified team for the rest of the series.