A boy named Evan Taylor (Freddie Highmore) lives in an orphanage, all the while believing that his parents are alive. He believes the music that he hears all around him is his parents communicating with him. Evan does not want to be adopted because he believes his parents are still alive.
Through a series of flashbacks, his parents are revealed to be a famous concert cellist named Lyla Novacek (Keri Russell), and Louis Connelly (Jonathan Rhys Meyers), an Irish guitarist and lead singer of a rock band, who spend one romantic night together and never see each other again.
While Evan has believed his parents have wanted him all along, his mother only lately comes to find out that her son is alive and sets out to New York to find him. His father has never forgotten Lyla and doesn't know about Evan! Both have since given up music.
Evan makes his way to New York City, where he is taken in by a man known as "Wizard" (Robin Williams), who houses various orphans and runaways, employing them to play music on the streets and taking a large cut of their tips. Evan immediately proves to be a musical child prodigy. Wizard enlists him and gives him the name "August Rush", convincing him he will be sent back to the orphanage if his real name is ever discovered.
After a raid by the police, Evan takes refuge in a church, where he again impresses with his natural musical talent and is enrolled at the Juilliard School as "August Rush". A work he composes is chosen to be performed by the New York Philharmonic in Central Park, but Wizard barges into a rehearsal, and Evan reluctantly follows him back to his life of playing on the streets.
Meanwhile, Lyla has discovered Evan's identity and has been living in New York searching for him. While there, she decides to resume her cello career and is chosen to play in the same concert which features Evan's piece. Louis, believing Lyla to be married, also returns to New York to resume playing with his former band and has a chance meeting with Evan in Washington Square Park.
The night of the concert, Evan finally chooses to run from Wizard in favor of performing at his concert. Louis races to the park when he sees Evan's pseudonym along with Lyla's name on a sign billing the concert. Evan conducts his piece, and at its conclusion, he turns around to see Lyla and Louis standing hand in hand.
He finally makes the connection that they're his parents.
