What is a summary of The Light Between Oceans by M. L. Stedman?

The Light Between Oceans is a novel by author M. L. Stedman that follows Tom Sherbourne, a man returning to Australia after experiencing the ravages of war on the Western Front. The story takes place in 1926, when Tom begins working as a lighthouse keeper at Janus Rock. The island features an isolated lighthouse nearly a full day away from the nearest town on the coast. Tom's wife Isabel comes to live with him on the island, but their peaceful life is tarnished by grief when Isabel miscarries twice and has one stillbirth.

Tom and Isabel are still grieving their lost children when a boat washes up on the island. They discover a dead man and a crying baby inside the boat and Isabel immediately decides to keep the infant and raise her as her own. Tom is unsure at first and feels obligated to report the dead man and the infant to the local authorities. He is ultimately convinced by his wife, who views the baby as a "gift from God," and they pretend that she is Isabel's stillborn child. Once Lucy reaches the age of two, the small family leaves the island and moves to the mainland, but the decision is not without consequences.

When Tom and Isabel reach the island, their dreams of a family are shattered once again when they discover that Lucy's birth mother is looking for her and that her real name is Grace. The couple's marriage is tested to its limits as they must come to a decision about their daughter's future and whether to confess the secret they buried long ago. Tom is filled with guilt and sends an anonymous note to Lucy's mother, telling her that her daughter survived. This launches a police investigation, which quickly reveals Lucy to be Grace. She is removed from the island and returned to the custody of her mother, Hannah. Isabel is furious with Tom, who covers for her by telling the police that keeping Grace was his idea. Even when he is jailed on her behalf and faces the threat of hanging, Isabel refuses to help him and attempts to frame him for the murder of Grace's father.

Meanwhile, Hannah finds that her reunion with her daughter is bittersweet. Grace resents being taken from Tom and Isabel and grieves their loss endlessly. She begs to be returned to the only parents she has known. Reluctantly, Hannah agrees to return Grace to Isabel if she will testify against Tom. Isabel ultimately decides that she is not able to betray her husband and confesses. He is released from prison after six months and she is sent to a mental institution. The couple moves to a small coastal town far away and Grace grows up with Hannah. Twenty years after Grace was found, Isabel dies of cancer and Lucy-Grace shows up at her funeral to make amends. The book ends on a somewhat hopeful note with Lucy-Grace saying she hopes to visit Tom again someday.