The fourth colourful and action-packed Victorian detective novel about the exploits of agent Mary Quinn. Mary Quinn and James Easton have set up as private detectives and are also unofficially engaged to be married. But when the Agency asks Mary to take on a special final case, she can't resist, and agrees.

Convicted fraudster Henry Thorold (from book one, A Spy in the House) is dying in prison. His daughter, Angelica, is coming to see him one last time. Mary's brief is to monitor these visits in case Mrs Thorold, last heard of as a fugitive in France, decides to pay him one last visit. But Mrs Thorold's return would place James in grave personal danger. Thanks to the complications of love and family loyalties, the stakes for everyone involved are higher than ever.