What is the main conflict in The Thirteen-gun Salute by Patrick O'Brian?

The two primary protagonists, Jack Aubrey and Stephen Maturin, are particular friends of long acquaintance. Aubrey has spent most of his life as an officer in the Royal Navy, eventually rising to the rank of post-captain. However, political enemies have contrived to falsely accuse him of financial crimes which has resulted in his ejection from the navy. Recently, Aubrey's accusers have in turn been discredited as traitors and have fled England for France; nevertheless, Aubrey remains a private citizen.