In post-World War I Western Australia, Tom Sherbourne, a war veteran, takes up the isolated job of a lighthouse keeper on Janus Rock. Seeking solace from his traumatic past, Tom finds comfort in the tranquility of his solitude. His life changes when he meets Isabel Graysmark, a vibrant and spirited young woman who persuades him to let her join in his remote life. They marry and attempt to start a family, but their happiness is marred by Isabel's repeated miscarriages, leaving her heartbroken and the couple emotionally strained.

One day, a lifeboat washes ashore carrying a dead man and a crying infant. Despite Tom’s adherence to rules and protocol, Isabel convinces him to keep the baby girl as their own and bury the man without reporting the incident, believing it to be a sign from fate. They name the child Lucy and raise her as their daughter, deeply cherishing their newfound family. However, Tom's growing guilt and a chance encounter on the mainland reveal devastating truths about Lucy’s real mother, leading to heart-wrenching moral dilemmas that threaten to unravel their world. The story delicately navigates themes of love, loss, and the far-reaching impact of choices on people's lives.