Well, that was garbage. Apparently it was a Marvel Comics crisis crossover and the culmination of a storyline that had been running since 2013. That explains some of its problems, but crossovers aren't inherently this rubbish.
So, the plot. There's a gas cloud (Terrigen Mist) that kills mutants (the X-Men), but gives other people superpowers (the Inhumans). It's floating around the Earth and getting bigger. The Inhumans see it as culturally important and there was some hoo-hah over destroying another cloud that ended in the death of Scott Summers (Cyclops). Now, though, the two sides are co-operating in looking for a peaceful solution. Hank McCoy (Beast) is running the scientific investigation and so loves the sound of his own voice that you're almost glad when, whoops, he's stumped.
SENSIBLE SCIENTISTS: would keep everyone in the loop about the scale and timeline of the threat.
IDIOTS (i.e. Beast): surprise their fellow X-Men one day with the news that they only have a fortnight before the world becomes uninhabitable for them. Has Beast told the Inhumans? Nope. Does anyone? Nope. Instead, they launch an unnecessary but oddly tame war in which neither side really tries to hurt the other. The Inhumans have no idea about the cloud's status and when they do eventually learn (very late in the book), they destroy the cloud themselves. D'oh.
Admittedly, the book has a (bad) excuse for this. Emma Frost's pushing for war because she wants revenge for the loss of her lover, Scott. "It's really about Emma and Scott," says someone... but I didn't buy it. As Emma's personal motivation, sure, but her colleagues should have been judging more rationally.
It's wall-to-wall bollocks. I found it barely readable, from its boring start to its empty finish. I'm clearly missing a ton of context by not reading lots of other Marvel comics from 2013 onwards, but sod that. It's a toothless "war" like a playground squabble, but with superpowers. The communication failures are so bad that people attack people who'd been trying to help them. Characters with multiple superpowers choose to attack with ones that their current opponent can resist, instead of using more powerful superpowers that would have made for a one-sided mismatch. (Black Bolt could survive a fist fight with the Hulk, but he loses to Emma Frost and Dazzler because he attacked them with his sonic scream. Jean Grey and the Cuckoos had a million options against Karnak, including the fact that Jean's a super-powerful telekinetic capable of restraining him physically... but instead they try to trap him in illusions, letting him use his ability to find the flaw in all things.)
To be honest, the most attention-grabbing thing about this book is simply the weirdness of the 2016 X-Men. Some of them are running around in old and young versions, thanks to time travel and a van that's bigger on the inside than the outside. (There are two Beasts. There's a young Scott, even though the old one's dead... for now. He'll be back. There's even a curvy female Wolverine called Laura, alongside the Old Man Logan from Mark Millar's dystopian alt-universe mini-series that got turned into a 2017 movie.) Former villains have joined the X-Men (Magneto, Emma Frost and possibly Sabretooth).
We also, unfortunately, have the Stepford Cuckoos. Wow, I hate that name. Why not have a hero called Indiana Skywalker 007 and a villain called Darth Blofeld? (I'm a massive John Wyndham fan and The Stepford Wives might be more iconic still. Grant Morrison's portmanteau name is respectful towards neither of those original works.)
Even the Fantastic Four's Human Torch is lurking in the background, as the boyfriend of the Inhumans' queen.
The book just doesn't make sense. It's also boring, but the central dilemma is fine and it could have worked if the plot hadn't needed everyone to be an idiot. Why didn't anyone use Storm? They're dealing with a cloud and she's got weather powers. Why did the X-Men attack in the first place, instead of letting Forge quietly use his cloud-destroying machine? If they hadn't made a fight out of it, the Inhumans might never have realised about Forge until it was too late.
Don't approach this book, obviously. The art's quite pretty, though.