Fast Five is a shining example of the Summer Action Movie, a film that has characters that you're nominally supposed to care about but you don't because it's not all that important, really, and the scenes where they're talking to each other are only there to give you room to breathe in between all the explosions and noise. Sure, it's bad, objectively, but if you sit there the whole time thinking about how bad it is, you're doing it wrong - you go to a movie like Fast Five because your brain has been working too hard lately and you'd like to give it a chance to shut off for exactly two hours and ten minutes.
So, okay, our movie opens with cars driving fast because Vin Diesel has been sentenced to 25 years in prison with no chance of parole, which we the audience know is bad because Vin Diesel is the Robin Hood-type criminal who steals and breaks the speed limit out of the goodness of his heart. Within two or three minutes, the prison bus he's in has flipped over, and he's on the lam.
Cut to Rio di Janeiro! We know it's Rio because the movie takes every opportunity to show us a sweeping establishing shot with Cristo Redentor as its centerpiece. The Third Male Lead and Vin Diesel's sister visit Zach Galifianakis, who I guess is maybe Vin Diesel's brother (?), who tells TML and Sis about how they can earn some easy money boosting some cars on a train. But things go bad! And some gummint agents who are watching the cars get "capped," and Vin is wrongly implicated in said capping. As a result, the United States sends Dwayne "The Rock" Johnson to capture him and bring him to justice.
Then a lot of stuff happens, and we meet our primary antagonist, a guy with slicked-back hair named Reyes. He wants one of the cars that was on the train because he hid a chip in it with secret directions to all of his secret drug money safehouses. I'm not sure why he needed a digital record about his own safehouses in the first place? Maybe he's super forgetful or something. Anyway. Vin and Company discover this chip and decide they want to hit all of these secret moneyhouses because getting all this money is the best way to buy their freedom. Someone utters the phrase "one last job," which means that, by default, the movie goes from being about cars that are super fast to being about an Ocean's Eleven-style heist.
And from there, it's just a super-stupid version of Ocean's Eleven. There's a montage where Vin mutters "we're gonna need a [blank]" several times, where [blank] can be a Guy Who Knows Computers or a Guy Who Is Super Sneaky, and even though they go through the motions of setting up specific roles for all of the characters, it turns out that all of them just drive cars really fast and furiously.
What follows is a movie cast in the mold of any post-Ocean's heist movie, except you can replace George Clooney's affable fast-talker with Vin Diesel's brawny mumbler. In fact, this movie is pretty much just Ocean's Eleven, except instead of pulling off a perfectly-planned, well-thought-out heist, imagine that George Clooney and Brad Pitt blew up the Bellagio with a bunch of dynamite and then shot every single person in between them and their ill-gotten gains.
That's basically it. There's nothing of substance to this movie by any stretch of the imagination - Vin Diesel mumbles some about family and stuff, but it all takes a backseat to explosions and Truly Muscly Men chasing each other across Brazilian rooftops. And, really, that's all it needs to be.