The proto-Muir Island X-Men are dealing with the escaped "Mutant X" (first hinted at in #104) - or "Proteus" as he calls himself - who it is dramatically revealed is Moira's son. He's killed, and will kill again - he burns up host bodies pretty quickly. The X-Men track him, worried he's going to go to a major city for noms. Moira takes aim and starts shooting, but is stopped by Cyclops before she can get the kill shot. Instead, he escapes and kills more people. Moira visits her husband Joe MacTaggert (er, I wonder if she's told Sean that she's not really a widow after all. Or about that Nobel Prize she's been keeping quiet. For that matter, how did Xavier manage to keep Moira a secret from the O5 all that time? I mean, I like the idea of Xavier having peers (the closest he got in the original run was I suppose Lykos!), but could this not have been done in a way that didn't imply such skullduggery). Joe is Proteus's father but doesn't know about it, and Moira retains just enough concern about her ex-husband - despite clear hints at it having been an abusive relationship - that she's prepared to warn him. Too late though, Proteus possesses Joe, and takes Moira hostage. Colossus takes him down in the end.
The reunion of Scott and Jean is slightly messed up by this Jason Wyngarde guy popping up again and inserting himself into her waking dreams. Carefully watching Wolverine's powers, I see that Scott uses his optic blasts to decelerate a falling Wolverine. These days he'd just let him hit the ground and heal.
Issue #125 is notable also for its little cutway to Magneto, wherein he reveals he had a wife called Magda once. This is a crossover with Avengers #186, which shows what happened to Magda - she fled to Wundagore and gave birth to Quicksilver and the Scarlet Watch, then vanished again. Her disappearance is probably the biggest unexploded bomb in comics. The image of her running away from Max, aghast at his powers (in this version he wants to rule the world from day 1 - there's nothing about a mob or a kid or the secret police) only to be perfectly OK with the High Evolutionary's talking animals is kinda hilarious.