Susanna, Lady Appleton, is passing time with her new love, Nick Baldwin, in Hamburg when she is approached by her old friend Sir Walter Pendennis. Distraught over the discovery that his late wife, Eleanor, was participating in a conspiracy to overthrow Queen Elizabeth and replace her with Mary, Queen of Scots, Walter beseeches Susanna to help him foil the plot. Since Eleanor's death is still a well-kept secret, Walter wants Susanna to take his wife's place among the traitors.

Overcoming her misgivings, Susanna soon finds herself impersonating Eleanor so that she can join the ring of conspirators. Attempting to deliver a vital message to the Earl of Northumberland, a key player in the drama about to unfold, Susanna wonders how long she can conceal her true identity. Will the plot against the Queen proceed unchecked, or will it be thwarted by Susanna's sleuthing?