If you haven't seen The Raid: Redemption (The first movie) here is a quick recap:
In the Raid, a crew of cops storms an apartment building filled with bad guys, gangsters, drug dealers and kung fu masters apparently. The main character, Rama (Iko Uwais), is one of the cops and also a bad ass brawler. It takes about five minutes for the building management to go over the PA system and tell all the tenants that is they kill the cops they will get free rent for ever... Insane violence ensues. Rama eventually finds out his lost brother is working for the bad guys in the building and they team up to survive the fight. If you like hardcore action then this movie is for you!

Rama getting down and dirty during a prison riot
Now that we are on the same page The Raid two picks up right where the first leaves off. Rama gets inducted into a small unit trying to seek out the corruption in the system, they insert him into the prison system so he may get close to a mob boss' son, Uco (Arifin Putra). During a brutal prison riot he saves Uco's life and the movie instantly teleports us two years later. Rama is getting released from the slam and is picked up by Uco and brought to see his father. Rama gets hired as muscle and becomes part of Uco's crew. The plot from here out (like the first movie) is kind of cookie cutter.
********** SPOILER ALERT! Skip the italicized portion! **********
Uco wants to take over the family business, his dad doesn't believe he is ready and too impulsive, Uco gets pissed and starts acting on his own, tries to take over the city, kills his dad and a bunch of Japanese gangsters then goes after Rama when he finds out he is a snitch. Everybody kills everybody. THE END.
The plot of this movie is not the reason to watch. The Raid offers some of the most hardcore and insane action put to film to date. The movie feels like a video game, each successive fight scene ups the ante in difficulty and brutality. It hurts to watch, even the most jaded person will be cringing a few times during this film. The choreography is intricate and intense and the Boss fight bad guys are interesting. Two of these are Hammer Girl and Baseball Bat Man. [No kidding, these are their names]
Unfortunately, they don't delve into their past but I assume these two are a brother/sister assassin team. One of those a bit to close for coworkers, not

Hammer Girl getting busy on a train
I think the best thing I can show you for this movie is what they threw out. A few weeks before the limited release the production sent out a deleted scene for the movie. Watch it here. After watching that consider this: They cut that scene out of the movie because they didn't need it in the face of all the other insane scenes they left it.