The movie starts with a shot of a man (Joshua) in front of a movie theater with a congregation, asking young "Brother William" if he would like to read a passage. In the middle of his reading, we cut to a movie theater showing "First Blood". In the back of the theater, young Lee Carter is smoking and filming the screen.
The opening credits roll while we watch Lee run home from the theater.
The next shot is of Will in a shabby shed, creating flipbook animations of planes and creatures. Before leaving for school, he glances admirably at a fancy watch (owned by his late father).
At the school entrance, Will's mom says goodbye to him, but not before noticing the watch and telling him to put it back wherever he found it. Will goes to a bathroom stall and adds to his many drawings. He also goes to get a drink of water and holds it in his mouth throughout class. The class is about to watch a documentary so Will is excused (due to his strict Plymouth Brethren, he isn't allowed to watch TV or films). He goes out to a hallway, where he spits his water into a fishbowl. Down the hall, we see Lee being yelled at and made to sit in the same hallway. Will passes the time with his drawings, while Lee bounces a ball (think Steve McQueen in "The Great Escape"). Lee throws the ball at Will's head, and tells him to fetch. Will grabs the ball and gets ready to throw it back. However, he turns to see Lee going through his drawings. In a bullying fashion, he keeps the book away from him. The two scuffle, and the fishbowl gets knocked over.
We next see the two waiting outside the principal's office. Will seems scared to death, while Lee looks bored. Lee said he'll take the blame in exchange for the watch Will is wearing. Will won't, so Lee starts to make up stories of how the school tortures you in the office. Will falls for it and says he'll pray for him. Lee puts on the watch, but skips out of entering the office, and sneaks out. While outside, Will sees a bus full of French exchange students. One French student (named Didier) comes across with a rock star look and starts to form his own little following amongst the English students (which we see short shots of throughout the movie, including a line of girls waiting to see if they're a good enough kisser to be with him). A teacher assigns hosts, while Lee pickpockets another teacher. He sees that Will has spotted him, so he fakes a lot of pain (from the 'torture' he endured). Will falls for this as well, and helps him to a bench. Lee excuses himself and goes to steal a bike (which happens to have a little wagon in the back). Since he believes Lee is hurt, Will offers to give Lee a ride in the wagon.
We see that Lee lives in a big mansion, connected to a nursing home. Lee tells Will that the watch wasn't enough payment for the "torture" he took, so he forces Will to agree to be the stunt man for a film he is making. Lee finds a crossbow and decides to open a piggy bank with it (on top of Will's head) for his film. Lee hears his older brother, Lawrence, coming. He makes Will hide, while Lawrence and his friends bully Lee and demand their copies of the Rambo film that Lee pirated. Lee tries to win his brother's approval by giving him the watch (it's his birthday). Lawrence doesn't even say thank you, leaving Lee walking back dejected. Meanwhile, Will is still hiding while "First Blood" plays in the room. He can't help but watch and is mesmerized by the action.
After Lawrence and his friends leave, Lee goes to find Will--only to find him long gone. Will runs home like he's running in the jungle (like an action star). He goes to his family's scarecrow and has a fantasy fight with it. That night, he dreams that he has the physique of Rambo. He even calls himself "Son of Rambow". He imagines that he saves his father (Rambo) from the scarecrow. His drawings now go from airplanes and mystical creatures to the militaristic setting of "First Blood". Later at school, while the class watches the documentary, Lee bumps into Will in the hallway. He orders him to meet him so they can finish the film.
Will lies to his mom and says he's helping a student who had an accident. He makes his way to Lee's, dressed up as Rambo. Lee makes him wait in the bushes as he finishes cooking something for Lawrence. Lee shoots scenes of Will being propelled in the air, sledding down a steep hill, being shot by a high pressure hose, and falling down through tress branches. Will doesn't seem to mind, since it resembles stunts found in action movies.
One day, before going to a prayer meeting, Will's mom sees his shoes need repair (Will purposefully damaged them the night before). Will says he'll catch up with them after getting them mended. The time needed to go to the menders buys himself time to film with Lee. We next see Will at the store, picking up a few things, and spots Lee shoplifting. Will decides to emulate him, and shoplifts as well. Lee looks through Will's book again, and decides to use his "Son of Rambow" idea as the movie. The part of Rambo is played by Frank, an elderly old man who lives at the nursing home. Frank doesn't follow the script, and starts complaining about the food he eats and the screams he hears at night. Lee decides to use this as some sort of improv, before a nurse kicks them out. The next scene is of Will jumping into a lake to "avoid gunfire". However, Will can't swim so Lee has to jump in and rescue him. The two share a laugh afterwards.
The boys dry off and talk about their families. We find out that Will's dad passed away long ago from a brain aneurysm. Lee's mom and step dad are usually out on business (usually Spain), leaving him alone with Lawrence (who acts as his male role model, despite how cold he acts towards him). Lee cuts Will's hand, as well as his own. The two hold hands to become blood brothers. The two bond that day with Will saying, "This has been my best day of all time".
That night, Brother Joshua comes by Will's home and speaks with his mom. He's concerned about the fact Will is missing meetings and wants him to get back on track. Will attends a church meeting about temptations for the whole day. On the way back home, Will looks out the window of the car and imagines his drawings coming to life in the passing fields. Joshua leaves Will's home later that night, and it's obvious he is trying to play a father-like figure in the household.
One day Lee snatches Will into an alley and demands to know where he's been lately (he's been missing film shoots). Will explains that his church has kept him pretty isolated and that he's not allowed to be friends with him. Lee is saddened and has Will drop by to at least pick up some of the things he left. Lee uses this opportunity to show Will a dog statue with kite wings (resembling a drawing Will made). Will gets excited and decides to continue the film anyway. However, the wind is too strong, and no matter what the boys do, the kite drags them along. It flies into a classroom, and sets off a fire alarm. A teacher pokes his head out the window to see that Lee is involved. Lee tells Will to take the camera and that he'll meet up with him later. Will can't find his book, and we see a group of students looking through it.
Lawrence picks Lee up from school (he's been given a week's suspension). Meanwhile, Will is lead into a church by a group of students (Didier's followers). The doors are closed and Didier acts theatrical in front of Will. It turns out the students shared Will's drawings with him. He wants to be a movie star ("just like Patrick Swayze"). We next see Will directing a scene (without Lee) of himself being attacked by ninjas. Didier (playing a character named 'Wolf') comes out of the bushes and saves him. We also see that Didier's 'men' are now shared by Will. The next day, Lee playfully ambushes Will only to lead to a scuffle with Didier's crew. Will explains who is who and has a chat with Lee. Lee is skeptical about Didier's involvement.
Everyone at the school likes to follow Didier's lead and trends. So now that Didier is interested in hanging out with Will and filming the movie, Will (and his film) have gained a sense of popularity. Lee is putt off by the popularity of both. Later that night Will is told a story by his mom. When she was a little girl she heard a song that she loved so much. She saved up to buy a record and a player to listen to the song every day. One day the elders came by to burn the record. She explains they did this to protect her and their way of life. Will agrees that he won't betray the brethren. However, later that night he sneaks out to meet Lee and the crew. Didier tries to make Wolf the central character in his battle with the scarecrow, which leads Lee to "accidentally" hit Didier in the head. Filming stops to get Didier back to his senses.
Early in the morning, Lee comes by Will's home. Lee apologizes for all the trouble he's caused, but explains that he wants the film to be just the two of them. Will won't come down to talk to him nor does he accept the gift given to him. The whole commotion wakes up Will's mom. She responds by having Joshua come by the house to take Will away for a few days. He tells Will that he's bringing the brethren shame, to which Will responds that he hates the brethren. He then breaks Joshua's hold and runs off. Will makes it to the filming location (an old construction site), where we see the whole student body participating. Lee comes by with his camera, and he argues over details with Will. The argument (including Will taking a verbal shot at Lawrence) leads to the two fighting, with Will throwing a rock, which cuts Lee above the brow. Everyone is on Will's side, which leads to Lee running off the set.
Didier drives a small jeep with Will in the passenger seat. However, he drops his cigarette and crashes, sending Will flying into a pit of oil and causing debris to fall on top of him. All of a sudden, Lee appears to remove the debris and pull Will out. He tearfully screams at him that despite his faults, Lawrence is all he has. He feels betrayed by his "blood brother" and says the only reason he saved him is that he wants his camera back. Will goes to get it, but hears a noise. He turns around to see more debris falling down. He warns Lee, but is too late as it comes crashing down. The next scene is of Lee being rushed to the hospital.
Later that day, Joshua comes by to talk to Will's mom about their future in the brethren, while Will cries in the shed. His mother comes by to comfort him. His little sister comes in and says she found something in the garden (the gift Lee had brought Will from before). He opens it up to find his father's watch. The next shot is of the French exchange students leaving the school, which acts more as a farewell for Didier. On the bus ride home, we see that Didier is actually treated mockingly as an outcast amongst his French peers.
We next see Will visiting Lee in the hospital (Lee has a broken leg). Lee however, doesn't want to see him. Later at Will's house, Joshua makes one too many comments about how Will is raised, so his mother finally throws him out. Back at Lee's, Lawrence sees his brother's tearful yelling (the camera was still on). He hears a knock at the door, and it's Will giving the watch back to Lawrence.
Lee is told he's being taken back home, but the ambulance takes him to the movie theater. The main feature is preceded by the short film "Son of Rambow". Lee laughs and enjoys the film as we see Will has edited and done some dubbing to minimize Didier's role, and keep the film more true to their original vision. He also adds a scene where Will knocks out a henchmen. The henchmen removes his helmet (revealing Lawrence) and tells the Son of Rambow to find his long lost brother one day and tell him that he's sorry if he hasn't been there enough for him. The film ends with a prayer of thanks for the friend he's made, while we see footage of the two becoming blood brothers. The audience loves the film, and Will surprises Lee in his seat.
The two leave the theater with Lee saying "This has been my best day of all time".
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