Complex sentence: He is sent to Portugal to monitor Edward, Duke of Windsor and Mrs. Wallis Simpson; when they move to the Bahamas, Mounstuart follows, playing golf with the Duke and socialising regularly until the murder of Sir Harry Oakes.
Split into simpler sentences: He is sent to Portugal to monitor Edward, Duke of Windsor and Mrs. Wallis Simpson; when they move to the Bahamas, Mounstuart follows. Mountstuart befriends the couple again, playing golf with the Duke and socialising regularly until the murder of Sir Harry Oakes.