"A Faithful Man" follows Abel (played by Louis Garrel), who as a Parisian journalist runs into his ex-girlfriend Marianne (a role portrayed by Garrel's real-life wife Laetitia Casta). They meet at his friend's funeral a decade after their breakup and Abel tries to win her back as he navigates a love triangle between himself, and his deceased friend's younger sister (played by Lily-Rose Depp).