Kevin Hart plays Calvin Joyner, class president and everything else in high school who now has a very ordinary life while others are promoted above him at his accounting firm. Dwayne Johnson is Robbie Weirdicht / Bob Stone, the really fat bullied guy from high school who's now incredibly buff. He remembers Calvin, because Calvin was the only person who was kind to him. On the eve of their 20 year high school reunion Bob reconnects with Calvin through Facebook and they go for drinks. And we learn very early on that Bob can fight really well. In fact Bob has some magical skills: if you're not looking at him he can move unbelievably fast (but the movie sees this as humour, not "magic"). Ordinary Calvin is horrified to be dragged into a world of spies and espionage that he doesn't understand, but Bob is overjoyed to be re-united with his best buddy from high school. In fact Bob's love of Calvin is genuinely creepy and feels like it could only be horrifying, but is of course played for humour instead.
Much of the humour is based on humiliation, with Calvin mortally terrified of everything going on around him and Bob acting like he's still a fat, bullied high school boy (but now with muscles). Not my kind of humour. The movie did still manage some laughs for me, so not a total loss.
In fact ... here's a quote from early on that had me in stitches:
Thug: Hey, how about this? Why don't you and your boyfriend apologize to Big Rick here and then go jerk each other off in the parking lot?
Calvin: That's, that's a lot...
Bob: Yeah. You're right, CJ. That's a lot of homophobia coming out of a very angry man. You need to go get that looked at by a trained professional. But, since you have escalated this whole scenario by bringing what I can only assume is an unlicensed firearm into this public place, endangering the lives of all these innocent people, I can no longer, in good conscience, walk away and jerk anyone off in the parking lot.