Having survived a long, desperate adventure in the Great South Sea, Captain Jack Aubrey and Stephen Maturin return to England to very different circumstances. For Jack it is a happy homecoming, at least initially, but for Stephen it's disastrous. His little daughter appears to be autistic, while his wife, Diana, unable to bear this situation, has disappeared, with the child looked after by the widowed Clarissa Oakes. Much of the story takes place on land, but soon Aubrey and Maturin are sent on a mission to the fever-ridden lagoons of the Gulf of Guinea to suppress the slave trade, but their ultimate destination is Ireland. There the French are mounting an invasion that will test Aubrey's seamanship and Maturin's resourcefulness, and the climax of the story is one of those grand, thrilling fleet actions on which the British Navy's supremacy was founded.