Ruso and Tilla have arrived back in Britain, newlyweds and in need of a place to live and a job for Ruso. Ruso's old friend Valens, from his army medic days, has found him one--as an investigator for the procurator, to locate the missing tax money, not to mention the missing tax collector, Julius Asper, from the town of Verulamium.
Asper's lover Cama (spelling optional since I listened to the audiobook), traveled the twenty miles from Verulamium to Londinium to report Asper's disappearance and what she believes to be the perfidy of the town leaders. She quickly winds up at Valens' house, giving birth to Asper's baby with Tilla as mid-wife.
Meanwhile, Ruso is tracking down Asper, and instead finding his corpse. That's not nearly so unpleasant for Ruso, though he's not pleased, as discovering that his old enemy Metellus is also interested in the case, and also expects a report from Ruso. Soon Ruso is off to Verulamium "to help the town council," and Tilla is also, accompanying her patient Cama and her baby. The tight bond between Ruso and Tilla, and the counterpoint of their disagreement about nearly everything, including marriage, relationships, and investigating crimes, ensures that they are often working at odds even as they have much the same goals in mind.
This is a nicely complex mystery, with enough but not too much Roman and British history layered in, and the continuing growth of Tilla and Ruso as characters. I've enjoyed every one of these stories so far, and I expect to continue doing so.