Thomas and another Special Branch agent, Gower, are on their way to a meeting with an informant who has vital information about a violent plot by political radicals, but when they come into view of the man, he unexpectedly runs. They pursue, splitting up in order not to lose him in the crowded and maze-like London streets, and catch up with him just in time to see him murdered by another radical, Wrexham. They pursue the killer, and could catch him, but Gower argues it's better to follow him back to whoever he's reporting to, and Thomas agrees. They wind up in St. Malo, France, having had no chance to call or wire Special Branch headquarters at Lisson Grove until after they are across the Channel.
So Thomas isn't there when his friend and patron, the head of Special Branch, Victor Narraway, is framed for embezzling the funds intended to pay off an Irish informant, and get him out of the country. Narraway is dismissed from his post, with possible charges of embezzlement to follow if he can't--without access to any of his papers and no agents to help him--prove his innocence fairly quickly.
We have two intertwined stories. Thomas Pitt tries to figure out what Wrexham is up to and what is the dangerous plot the dead informant, West, had intended to warn them of, while gradually realizing how little he knows his subordinate, Gower. Victor Narraway visits Charlotte Pitt to let her know where Thomas is, and the dangerous situation. (Since Thomas is Narraway' protege, and has risen fast despite being relatively new to Special Branch, Narraway's enemies are likely to be his enemies.) Of course Charlotte, realizing that Narraway has to be cleared for her husband to keep his career, insists on accompanying Narraway to Ireland to investigate his framing from that end. It's the first time in a while that Charlotte has been able to participate in an investigation, but Thomas isn't with her, and she doesn't have friends and connections in Dublin society as she does in London. And who can she and Narraway and, over in France, Thomas, trust? No one.
This is another nicely executed adventure for Charlotte and Thomas.
I borrowed this book from my local library.