As Rachel McKnight, a devoted member of a fundamentalist Mormon community in Utah, celebrates her fifteenth birthday, she experiences a life-changing moment prompted by an unexpected auditory revelation. The serendipitous find of a blue cassette tape featuring a cover of "Hanging on the Telephone" by an unknown rock band ignites a deep-seated fascination within her. Shortly after this discovery, when she finds herself pregnant, Rachel believes that her miraculous conception is a divine act, reminiscent of the Virgin Mary's immaculate pregnancy. However, as the reality of her situation begins to unfold, her community turns against her, scapegoating her brother, Mr. Will, for her predicament and driving him away.
With the looming threat of a shotgun marriage arranged by her father, Paul, Rachel makes a bold escape to Las Vegas with Mr. Will, who has been residing in the family’s pickup truck. Enveloped by the neon glow of the city, Rachel is drawn into the vibrant world of a group of skaters who live and create music together. She is particularly intrigued by one skater who sports a shirt marked with a cassette symbol. As she becomes increasingly involved in their lives, Mr. Will returns, urging her to admit that she has been intimate with someone else, thereby securing his place back in their community.
Rachel's bond with Clyde, one of the skaters, intensifies as they share tender moments amidst the chaos of the city. However, when Clyde proposes marriage and suggests they embark on a quest to find the mysterious man featured on the cassette tape—who Rachel believes is the father of her child—she finds herself torn between the yearning for independence and the craving for familial ties. As the tensions rise, a failed break-in at Clyde's parents’ home leads to Mr. Will being apprehended, pushing Rachel to flee further from her past.
In a moment of desperation, Rachel reflects on her mother’s bedtime stories and follows a red Mustang, eventually uncovering the astonishing truth that the car’s owner, Tim, is the very man who recorded the tape and her biological father. This significant revelation propels her onto a transformative journey that challenges her faith, identity, and perception of reality.
Ultimately, Rachel finds the courage to return to her community, leaving behind the upheaval of her recent experiences. At the same time, Tim seizes the chance to rescue Mr. Will from his plight, and they set out to reclaim Rachel. Their adventure unexpectedly intersects with Clyde, whose van has broken down on the highway. With Tim at the wheel and Mr. Will by his side, they rush to the church as Rachel prepares to take sacred vows. In a dramatic turn of events, they arrive in the iconic red Mustang, rescuing Rachel from her uncertain fate. Back in the community, Mr. Will's confession, captured on an old blue cassette, earns him a chance at redemption, while Rachel holds firm in her belief that God is the true father of her child. As time goes by, Clyde and Rachel enjoy a serene life together at the beach, walking hand in hand while her pregnancy blooms, their love deepening with each swell of the waves.
