40-year-old Antoine is losing his footing in life. When his marriage falls apart, he becomes obsessed with his mother's unexplained death in a drowning accident when he was 10 years old. The obsession is about to destroy his relationship with his sister Agathe, the only one close to him. Antoine's therapist forces him to confront the authoritarian father who banned any talk about his mother's death, and find out the truth. Boomerang is based on a book by Tatiana Rosnay.