What is the main conflict in The Surgeon's Mate by Patrick O'Brian?

Jack Aubrey, Stephen Maturin, and Diana Villiers enter Halifax, Nova Scotia, aboard the victorious HMS Shannon. They enjoy a heroes' welcome and then settle down to await transport to England. Maturin passes the time wondering about Villiers while Aubrey pursues an extramarital affair with the flighty Amanda Smith. Meanwhile, Villiers frets about her pregnancy by Harry Johnson. Finally, their transport sails to England, pursued relentlessly by privateers in the employ of Johnson. In Portsmouth and London, the three characters settle down to a period of relaxation and recuperation, Maturin meeting with his friend and confidant Sir Joseph Blaine.