The two are invited into a rager (Kelly brings the baby monitor with them, so I guess it's ok that poor infant Stella is left alone all night multiple times in this movie) and at first have a great time, but then when the noise continues for days on end, resort to calling the police for the sake of the baby they occasionally are concerned about. This leads to all out war between them and the frat, as one prank escalates to another as Mac and Kelly try to get them kicked out however they can manage it. That may seem like a lot of setup, but trust me, all of that only takes about 10 minutes before the war starts, and from that point on it rides the same joke pretty much into the ground until the movie's end. And as I said, some of the scenes are good (I liked when they pay a tortured freshman pledge to wear camera sunglasses into the frat) and some are just stupid, like the guys stuffing air bags into Mac's work place desk chair that sends him flying into the ceiling with some really cheap CGI effects that are deployed three times and are distractingly awful each time.
Seth Rogen brings his usual, pop culture obsessed, riff-heavy self to play here, and I'll admit I've always enjoyed his everyman persona, but once again this crew falls down in terms of developing the female character. Rose Byrne is fine in the movie, and clearly they wanted to draw her into the proceedings by making her a participant in the prank battles, but instead of giving her a personality of her own, what they've done is turn her into the ultimate cheerleader for her hubbie, parroting everything he says and nodding along to all his plans, so that the clear implication is that the perfect wife for any of the schlubby Apatow gang is a non-nagging woman who likes everything they like without the slightest deviation or individuality of self. Ugh. As for the frat guys, Dave Franco continues to be a winning and charismatic presence on screen in these movies (could he possibly turn out to be the greater Franco?) while the sculpted Ken doll that is Zac Efron makes the perfect fraternity president who always seems to be giving off a slightly psycho vibe in the intense stares thrown at every peripheral character on the screen.