It is London 1662, and plots abound against the king. Oliver Prior, haunted by the death of his sister when still a child, enters the world of espionage. Beatrice Short's goal is to go on stage, but she must work as a servant. While cleaning, she finds ciphers and invisible script. After the king's undersecretary finds her snooping, he blackmails her into going undercover as a spy.
While Oliver and Beatrice bond to discover the backbone of insidious schemes to kill the king, they learn who runs the plots. He is a man steeped in hate, and he must be put down.