Ray, in what I can tell is going to become a pattern, provides our entry into the story, through her psychic link with baby Nathan. She bolts, and the rest of Excalibur go follow: Cap carrying Nightcrawler and Meggan taking Kitty. One of Claremont's more successful attempts at outright humour follows, as the practicalities of a multi-hour flight over the Atlantic and what this mean for bladders are considered. ("Can't you just phase?", says Cap, to Kitty's disgust.)
When they arrive in New York, stuff is weird, in a way similar to and yet more lighthearted than the Inferno issues of the other X-Books. Ray has been nobbled, and a demonically-influenced Meggan provides our main antagonist. There is some fighting. Cap goes bad too; Kitty has to use the soulsword to free him. Oh. She has the soulsword. That was probably a bad sign, right?
The basic premise of Excalibur: i.e. that Kitty and Kurt think that the X-Men are dead, necessitates that there can't be any crossover with the other titles, so once they're here, all they can have is a different viewpoint of the same apocalypse happening, not meet any of the other teams, and not then contribute towards the overall resolution (because that's already being taken care of in too many titles already). Should have stayed at home, really.