What is the theme of Mara Daughter of the Nile?

Mara Daughter of the Nile is about a young slave girl who is sold to a master who wants her to be a spy for the queen. However, on the ship to her destination, she meets another man who believes she has run away from her master. This strangers blackmails her into being a spy for him. In the end Mara must choose which side she is on a matter made more complicated because she falls in love with one of the men in her life.