At the end of 2008's Rambo, John Rambo was seen walking towards the house of the mysterious "R. Rambo". You probably assumed that R. Rambo was his father or something; well, it is, and Sylvester Stallone plays both roles using state-of-the-art split-screen technology. R. Rambo is like "Hey son, where have you been for the last 50 years?" and Rambo the Younger tells him about his most recent adventure in Burma, where he killed like 400 people and how awesome that was, especially on Blu-Ray Disc. Rambo Dad tells him that's no excuse for not visiting more often, "Your homosexual twin brother Paul Rambo visits me all the time." And just then, Paul Rambo (also played by Stallone) enters and is like "Oh my god, what's up bro" And then John's sister, Mary-Anne Rambo (Stallone again) enters and is like, "Hey everybody, I'd like you to meet my new fiance, Rocky Balboa!" So Rambo and Rocky (also Stallone, of course) meet and shake hands. The handshake might have to be done with CGI or something.
Rocky & The Rambos talk for a while, when suddenly the doorbell rings. The entire Rambo Family is already there, so who could it be?? John Rambo opens the door to find a flaming bag on the doorstep. He stomps on it to put the fire out, and of course gets dog shit all over his boots. Then he has a flashback to Vietnam, where one of the Vietnamese guys did the same thing to a buddy of his. He remembers holding his dying, dogshit-covered buddy in his arms. Then, a couple of teenagers jump out of the bushes and are like "Take that, Rambo!" and they ride away on their bikes. Rambo takes out his machete and gets ready to chase them. The rest of the Rambo Family try to talk him out of it, but John's all "They drew First Blood" and, seeing that there's no stopping John Rambo, Rocky Balboa tells the family he'll go with him to make sure nothing too fucked up happens.
So Rambo & Rocky chase the Rowdy Teenagers through the jungle. At one point the kids throw water balloons at R & R, and they have to take cover behind an old tree. One of the balloons hits Rocky and he's like "Man, I think that balloon had piss in it! Now I got piss on my Boxing Jacket." This causes Rambo to have another Vietnam Flashback. Then they start chasing the kids again until they get to their hideout. R & R are up in a tree, spying on the kids, and they see them walk into a cave. Five minutes later, they exit the cave with fresh bags of dog crap and piss-filled balloons. They jump out of the tree and grab the kids, threatening to beat the shit out of them. The kids cry that they're just following orders, and their leader is in that cave. R & R let the kids go, but as they're walking away one of the kids throws a balloon at Rambo, so Rambo quickly dives out of the way and throws a knife at the kid. The knife hits the balloon in mid-air, causing the balloon to splash all over the kid so now the kid has his own piss on him. Also the knife cuts his head off.
Ramby & Rocko enter the cave. As they're walking in, Rocky's like "Hey, that was pretty fucked how you killed that kid." And Rambo's all "Sometimes in war, you gotta break a few eggs to make an omelette." I mean he actually says something better than that, but you get the idea. They keep making their way through the cave, when suddenly they're attacked by a group of kids with slingshots. Rocky gets hit in the eye with a rock, and Rambo jokes "I guess that's why they call you Rocky!" which doesn't actually make sense, but Rambo's not a comedian so what the fuck do you want. Since the kids are only slinging small stones and rocks at them, Rambo easily walks around the room and kills them all with his machete. Rocky doesn't want to kill anyone, but some fat kid comes up and starts shooting him with a Super Soaker, and finally Rocky has had enough. He punches his fist right through the fat kid's fat face. Rambo sees this and says "Looks like you just broke a few eggs..." Rocky replies, "Now let's go make a fuckin' omelette!" They each pick up a Super Soaker and bust down the door to the Cave Boss's office.
The Cave Boss is sitting in a big chair with his back to Rocky and Rambo, so you can't see who it is yet. He's like "So, it appears you finally discovered my secret cave, Mr. Rambo." Rambo's like "I had a little help." and Rocky says, "Yeah, I'm here too, ass-fuck" The Cave Boss says "I pity the fool who teams up with John Rambo!" Then Rocky realizes who it is. The CB turns around and we discover that it is Mr. T from Rocky III. But Rambo doesn't even know who he is, because he wasn't in that movie. Mr. T explains, in a 25 minute monologue, that he was the mastermind behind all of the events that transpired in the previous Rocky and Rambo films. We see some clips from First Blood with Mr. T digitally inserted into the background. Anyway, Rocky and Rambo shoot Mr. T with their Super Soakers, and Mr. T's like "Nooooooo" until he's totally Super Soaked. Then Rocky and Rambo both punch Mr. T in the face and the same time so Mr. T's head explodes. Rambo then looks at Rocky and says "Welcome to the family." They high-five and the credits roll over a freeze-frame of them high-fiving.