After Lady Rebecca Jamison, a mathematical genius, saves her family from financial ruin by secretly investing in railway stocks on the London stock exchange, a greedy syndicate, desperate for Becca's calculations and predictions, murders her friend and threatens the Jamison family, forcing Becca to beg assistance from her childhood friend, Cayle St. Martin. 

The newly titled Duke of Sherwyn has returned to London after five years on the continent extending his family's shipping interests. He's shunned his privileged London life and his father's unbending attitudes, and becomes committed to employing the spying tactics he learned on the continent to help Becca indict the syndicate—and using his skills as a lover to seduce her into his bed. 

But how will Cayle be able to convince Becca, a determinedly self-sufficient spinster, that he can be more to her than just a protector?