Caterina, illegitimate daughter of the Duke of Milan, is trained in military strategy and the use of arms, and betrothed at ten to Girolamo Riario, the Pope’s ruthless nephew.

When the Pope dies and his enemies turn on her family, the 21-year-old Caterina, heavily pregnant, seizes the arsenal of Castel Sant’Angelo to sway the election of the next Pope. Forced back to provincial Forlì, she wins the loyalty of Tommaso, castellan of its fortress, and the barber and writer Andrea, two friendships that will shape her life.

When her husband is murdered in a conspiracy, Caterina is taken prisoner with her children, but talks her way into the fortress and organizes the resistance. Regent for her son, she secretly marries a stable boy, loses him to another conspiracy and takes a terrible revenge. And she becomes the only ruler in Italy to stand against Cesare Borgia and the French, very nearly capturing Borgia himself, before a year in prison and a daring escape to Florence.