Who's David? He's been mentioned before, and a facsimile note of Moira's at the end of #25 gave the full details. He's Charles Xavier's son, by Gabrielle Haller, who was mentioned in #1. Moira has been looking after him. Recently, he has developed scary psionic abilities that he can't control, and although she agreed with Gabrielle to keep things quiet, she's left with no choice but to ask Charles for help.
Xavier, Moira, Gaby and the New Mutants (really, there's no need for the New Mutants to actually have come, but it's their book still, technically) go to Scotland, and check things out. This is a homecoming for Rahne, and she bumps into Reverend Craig, who curtly dismisses her. Later, Moira makes a clumsy attempt at bonding with Rahne, who manages to call her "Mum". In X-Factor spotting, Jamie Madrox makes his first appearance since Uncanny #129 here.
The book addresses the ethics of Charles's relationship with Gaby before he's presented with incontrovertible evidence in the form of David's paternity. This is good. He unconditionally apologises, saying it was "criminal". Which is not an exaggeration. She brushes it off, of course, the story isn't about that, and this is about the minimum effort it has to put into addressing it without pretending it was acceptable. Still, why even bother doing this, if the intent isn't to portray as Xavier as a severely unethical problem. His bastard son doesn't have to be by a former patient, and we could write off #161 as a cheese dream.
So, the story eventually delves into David's mind. Charles and Dani enter his dreamscape, to see what's going on there. Sienkiewicz's art is perfect for this, just as it was for the "Demon Bear" arc, and Warlock. What they discover is that he has multiple personalities, each with their own different powers, since a traumatic experience with a character sensitively referred to here as "the Arab" (in fairness, this is a plot point). This is called schizophrenia by the text which I could rant about, but even in 2012 storytellers don't seem to understand the difference. I blame the etymology mostly. Apart from the terminology and mental health fail, this is quite a novel idea, I don't think it had been done to this extent before. The resolution: that the Jemail was trying to fix things all along, and is finally allowed to, is a good one. He doesn't manage to reintegrate all the personalities, of course, which'll provide no end of hooks for terribly repetitive stories about a chap whose (a trend, I am happy to say, that Si Spurrier and Tan Eng Huat's X-Men: Legacy shows every indication of avoiding.)
Of course, this isn't allowed to be self-contained. In a cutaway to the the Massachusetts Academy, Emma is pissed with Empath for messing around in #193, and possibly ruining her manipulation of Firestar. We get the idea that Emma Frost is old colonial aristocracy for the first time. Meanwhile, Magneto and Lee Forrester kiss, apparently get added to the chart, and Magneto explains about Magda to her. This is an expansion from what we learned back in Avengers #186 - there's mention of saving them from secret police, but not, you will note, anything about Anya, as she hadn't been invented yet. If she had been, it would have been a very good time for Claremont to mention her, what with Charles's son being in the issue.
The episode of Star Trek referred to in #29 is "The Managerie", if you're wondering.