10-year-old Bart Millard lives with his mother and abusive father Arthur in Texas. One day his mother drops him off at a Christian camp where he meets Shannon. Upon his return from camp, Bart finds his mother has left and movers are removing her belongings. He angrily confronts his father, who denies that his abusiveness was the reason she left.
Years later, in high school, Bart and Shannon are dating. Bart plays football to please his father, but is injured, breaking both ankles and ending his career. The only elective with openings is music class, so he reluctantly signs up. Initially, Bart is assigned to be a sound technician, but after overhearing him singing, the director casts him in the lead role in the school production of Oklahoma!. Bart overcomes his reluctance and gives an impressive performance, but does not tell his father, who finds out the night of the show when he happens to see a flyer for the show in a diner. Arthur suddenly collapses in pain, and finds out he has cancer, which he hides from Bart. The following morning, Bart antagonizes his father, who smashes a plate over his head. At church, Shannon sees the blood and presses Bart to open up, but he responds by breaking up with her, and leaves town to seek his fortune in the city.
He joins a band in need of a singer, and convinces Christian music producer Scott Brickell to manage the band and secure a showcase in Nashville. Bart surprises Shannon and invites her to tour with the band, and is confused when she flatly refuses. In Nashville, Brickell introduces Bart to established artists Amy Grant and Michael W. Smith, but is unable to convince several record execs to sign the band, who do not believe the band, now performing as “MercyMe”, is good enough. Devastated, Bart quits the band, but Brickell perceives that Bart needs to resolve issues in his personal life, so Bart reconciles with the band and asks them to wait for him, and leaves to return home.
Bart returns home late at night and is confused to find that his father has prepared breakfast for him the next morning. His father claims to have become a Christian, but Bart is skeptical and refuses to forgive him, and leaves. In anger and despair, his father smashes his old Jeep, which he had asked Bart to help him restore. Bart attempts to drive away in his father’s pickup, but discovers the terminal cancer diagnosis, and returns to his father. He finally forgives his father, and the two form a deep bond, but Arthur soon dies of his illness.
After Arthur’s funeral, Bart rejoins the band and writes “I Can Only Imagine”, and also calls Shannon and apologizes for the first time since their breakup. Brickell sends the demo tape to several artists, including Grant, who, deeply moved by the song, asks to record it herself as her next single, and Bart, who just wants the song to be heard, accepts. On stage, Grant begins the song, but can’t bring herself to sing it, and calls Bart on stage from the audience to sing it himself. Bart’s performance earns an enthusiastic ovation, and he reunites with Shannon, who was also in attendance. The band releases the song as their first single, achieving success on both Christian and mainstream radio.