Milla Finlay, a 16-year-old schoolgirl, faces the harsh reality of her recent cancer diagnosis. One fateful day, while waiting for her train, she encounters Moses (Arka Das), a 23-year-old who quite audaciously asks her for money. Captivated by him, Milla introduces him to her parents—Anna (Essie Davis), a passionate musician, and Henry (Ben Mendelsohn), a dedicated psychiatrist. Despite their discomfort with the significant age gap, they reluctantly allow the relationship to flourish due to Milla's fragile health condition.
One night, Anna's instincts lead her to discover an alarming scene: Moses attempting to pilfer prescription drugs from their home. When awakened, Milla and Henry contemplate calling the police, but Milla's heartfelt plea for compassion sways Anna, who acknowledges the happiness Moses brings to her daughter. Subsequently, she cautions Moses to maintain distance from Milla, hoping to protect her.
Undeterred, Moses continues to make his presence felt at Milla's school. After a determined search, Milla finds him, leading to exhilarating drug runs and wild parties, culminating in a night spent on a rooftop. However, heartbreak ensues when Moses vanishes, prompting her concerned parents to take her to the hospital.
As time progresses, Henry and Anna realize that they cannot effectively prevent Milla and Moses' relationship. They become more accepting, allowing Moses frequent access to her. Yet, when Milla falls ill and Anna uncovers that Moses has pilfered her medication, a rift occurs. Milla feels betrayed, suspecting Moses of exploiting their family’s drug supply, leading her to kick him out.
In a surprising twist, Henry tracks down Moses and offers him a place in their home, promising him drugs in exchange for keeping Milla happy. This unusual arrangement works for a while until Milla learns of her father's deceitful drugging of Moses. Anger ensues, and Milla demands Moses leave, though he later returns, attempting to combat his addictions to prioritize their relationship.
After a joyful 17th birthday celebration, Milla confides in Moses about her unending pain and the approaching end of her life. In a moment of desperation, she implores him to end her suffering by suffocation, a request he tragically cannot fulfill. Instead, they share an intimate moment, their first time together.
The following morning, as Anna tends to Milla, she realizes with heartache that Milla has transitioned from life during the night. In a poignant flashback, Henry reminisces about a serene day at the beach with Milla. She confides in him her acceptance of dying and urges him to look after Moses. With love and sorrow, Henry promises that he and Anna will find their way through this impending grief.
