This is supposed to draw a line under the Madelyne Pryor/Jean Grey thing, but only leaves more questions open.
Mastermind has been working behind the scenes for a few issues - causing Rogue to leave Mystique and join the X-Men, Mystique to have strange dreamcs. This is seemingly with the intent of dividing and distressing the team. Rogue joining caused Binary - possibly their strongest associate - to leave. Wolverine's wedding collapsing wasn't fun either - and that seems to have been related. Cyclops of course is off with his new girlfriend, flying planes (he does seem to have a thing about women and transport, doesn't he? Something to do with his mother?)
So. The Plan. Mastermind uses illusions and mind control to
make Madelyne appear to manifest as Phoenix, which weirds
Scott out. Then, some hours later, Scott drops out of the
sky at Westchester, and after Professor X is conveniently
incapacitated by Cerebro backfiring (otherwise this wouldn't
need to be a double-size issue - #175 is the official 20th
anniversary issue - did I mention that? - and is the first
regular issue of X-Men to cost as much as a dollar! A whole dollar!)
Soon enough he's repeating himself and Scott appears
as Dark Phoenix (as Jean/Phoenix, that is, not the Scott/Phoenix
that he becomes in AvX #11 - I'm writing this the day after
that issue came out and the irony is not lost on me), and the
team immediately start attacking him. Although Xavier has been taken out, Cyclops is eventually able to drug Rogue (using the Danger Room, which can casually synthesise tranquilisers when simulating environments - the TNG holodecks owe a massive debt, it seems), and get her to nab a bit of Xavier's power. She confirms they're telling the truth, they take down Mastermind, and Madelyne is saved.
What is Mastermind's actual goal here? Simple revenge, he says.
They buy that - there've been more than enough villains
after them for that. I don't think he'd been planning
anything so elaborate when he'd been following them around, mind - he's been doing strictly small-scale messing around with them
until - as he notes - he found that Madelyne was the spitting
image of Jean. Which he's taken full advantage of, and them some. "Mastermind" is a boast that nobody with Moriarty-levels of evil coordination would make. He's left alive - although Storm is a rather more trigger-happy than she used to be, and is not pulling punches she would have previously, she still restrains Wolverine from outright killing him.
With all that done (although there's still the business of the
plane crash, and no records of Pryor existence before a certain date - surely they're not blaming Mastermind for that, too?), Scott and Madelyne get married. Just like that. Bit soon, isn't it? It'll never last.