This film presents a heartfelt coming-of-age story centered around four orphan boys and how their lives evolve during a single Christmas holiday in the late 1960s Australia. The narrative unfolds as four boys from a Roman Catholic orphanage - Maps (Daniel Radcliffe), Misty, Spark, and Spit - all born in December, are sent for a holiday stay with Mr. and Mrs. McAnsh at the beach.
During their time there, they encounter a man known as Fearless, who claims to be a daredevil motorbike rider in the local carnival, and his wife, Teresa. While Misty, Spark, and Spit quickly form a bond with Teresa, the eldest, Maps, feels reluctant to engage with her. Instead, he finds solace in the company of an older girl named Lucy, who is visiting her uncle at the beach. Maps often sneaks away to a secluded area with unique rocks to meet her.
As the days pass, an incident occurs where the orphans peek into Fearless' house and spot Teresa disrobing. Misty, being the most devout among them, throws a rock at the wall to divert their attention but later sneaks a look through a door, only to find it leads to Mrs. McAnsh, who is suffering from breast cancer.
One fateful night, Misty overhears Fearless discussing the possibility of adopting one of the orphans, igniting hope for a family among the boys. He keeps this exciting news to himself until he comes clean to a priest visiting for confessions. As the other boys grow impatient, they force the information out of him under undue pressure. Misty, Spark, and Spit eagerly vie for the affection of Fearless and Teresa, while Maps remains indifferent, questioning, > "What's the big deal about parents, anyway?"
Maps enjoys a romantic encounter with Lucy, who urges him to remember her as his first love, but the next day she leaves the beach to return to her father. Heartbroken, Maps seeks out Fearless to confront him but discovers that he is not the impressive rider he claimed to be; instead, he merely performs animal cleanup at the carnival. Betrayed, Maps destroys a heartfelt painting made by Misty depicting their hope for a family, leading to a fierce conflict that fractures their friendship.
Fearless later finds Maps in the cave at Remarkable Rocks and reveals to him the painful backstory that led to his and Teresa's desire to adopt. He explains the motorcycle accident that left Teresa unable to have children. As they reconcile, Maps learns that Misty has plunged into the sea and is drowning. Despite his inability to swim, Maps leaps in to save him, and together they encounter a vision that may signify their impending fate. Just as they reach out, Fearless rescues them, and the boys reaffirm their friendship.
The next day, a monumental announcement awaits them at Fearless and Teresa's home: they plan to adopt Misty. However, rather than leaving his friends behind, Misty realizes that they are truly his family and chooses to stay with Fearless and Teresa.
Decades later, an older Misty returns to the beach with Maps' ashes, who had served as a priest in Africa, and Lucy's ring—a memento from their shared holiday. He meets Spark and Spit, and together they honor their memories by scattering the ashes and the ring into the wind, celebrating their cherished moment as "The December Boys."
