Two people meet in a crowded London theatre one night in 1785. They quickly become lovers - and slowly discover that neither is what he/she at first appeared to be. As a spy for the Venetian state, Mimosina trades in dangerous secrets, and Valentine Greatrakes, the mastermind behind London's quacks, in elaborate lies. As Valentine pursues the murderer of his right-hand man, he gets closer to the truth of his lover's background - and both are forced to admit the similarities between the different kinds of contraband they trade in.