What is the main conflict in The Yellow Admiral by Patrick O'Brian?

Jack Aubrey and Doctor Stephen Maturin return home to discover that much has changed in their absence. Jack finds that he has been promoted to Commodore and given a new mission to stamp out the slave trade on the west coast of Africa, while Stephen discovers that his enemy the Lord of Habachtschal is conspiring to bring treason charges against him for his previous conduct as a British spy and for his support of Clarissa Oakes and Padeen Coleman.