This issue starts with Sue unconscious and sinking to the bottom of the Pacific ocean. She was out sailing and got washed overboard.
The story has her team-mates trying to find her, and her meeting Namor (who gave her a necklace enabling her to breathe under water). They fight Lady Dorma who apparently runs Atlantis on Counter Earth (Earth 616’s Dorma has been dead a long time).
The team eventually catch up with Sue and Namor and help him escape Dorma.
So: A one-off issue starring the actual Fantastic Four, with no fake members or weirdoes, and their longest-running frenemy, Namor. And the only alternate/future Earth character is Dorma.
No wonder this is the best issue of Claremont’s run. It’s also the last.
Thank God.