In December 1918, Tom Sherbourne, a traumatized and withdrawn World War I veteran, begins his new role as a lightkeeper at Janus Rock, a remote lighthouse off Australia’s coast. He soon falls deeply in love with Isabel Graysmark, a local woman, and they marry, but their happiness is overshadowed by heartbreak as Isabel endures two miscarriages within two years, leaving her fearful of never becoming a mother.
After Isabel's second loss, an unexpected event occurs when a rowboat washes ashore, carrying the body of a dead man and a newborn baby girl. Aware of his legal obligations to report the situation, Tom ultimately finds himself torn. Driven by Isabel's desperate wishes to avoid the orphanage for the infant, he makes the fateful decision to pass the baby off as their own daughter. They name her Lucy and bury the deceased man on the island.
As time passes and Tom and Isabel’s lives begin to stabilize, a trip to the mainland for Lucy's christening leads Tom to a shocking realization. He encounters Hannah Roennfeldt, who mourns at a grave marked with the names of her husband, Franz Johannes Roennfeldt, and their baby daughter, Grace Ellen, both lost at sea on the same day they found Lucy, April 27, 1926. Disturbed by the coincidence, Tom anonymously writes to Hannah, informing her of her husband’s death while assuring her that her daughter is safe and cared for.
Years later, the now idyllic life of Tom, Isabel, and young Lucy is shattered when accusations arise against Tom regarding the fate of Hannah's husband. Struggling with guilt and moral distress, he sends Hannah a small silver rattle that Lucy was found with, which ultimately leads to his arrest, as it was recognized in connection to a reward for information on Franz’s murder. Obscured under pressure, Tom courageously takes the blame, declaring that he had pressured Isabel into complicity. Consumed by fury and heartache, Isabel severs ties with Tom.
As Lucy is returned to her biological family, she inexorably rejects them, having no recollection of her early life with Tom and Isabel. Her need to return to the lighthouse and her "real parents" drives her to run away, prompting a desperate search. Eventually, Lucy is found and brought back to Hannah, who realizes that Lucy truly belongs with Isabel instead.
In a twist of fate, Hannah promises to return Lucy to Isabel if she will testify against Tom. Just as he is about to be taken for trial, Isabel discovers a letter from Tom expressing his burden of guilt, revealing that he believed he didn’t deserve the happiness he had with Lucy. Torn by Tom’s words, Isabel takes a brave leap and confesses the entire truth.
Moved by their sacrifices, Hannah decides to advocate for Tom and Isabel during the trial. Meanwhile, Lucy begins to form a connection with her biological mother, resulting in a compromise to name her Lucy Grace to honor both families.
Fast forward to 1950, an adult Lucy Grace Rutherford reappears in Tom's life, bringing her baby son Christopher along. For the past eighteen years, she had respected the promise to keep distance from the Sherbournes. Isabel has passed away, still haunted by her past choices. Tom offers Lucy Grace a heartfelt letter her adoptive mother penned for future contact. Overcome with emotion, Lucy expresses her gratitude to Tom, her true father figure, for nurturing her during her formative years at Janus Rock. Their reunion is poignant, and as they share a heartfelt embrace, Tom finds peace, content with the life he lived and the love he once shared.
