The title refers to the execution of criminals in the arena, by trained lions.

As part of his attempts to earn enough money to buy himself into the upper middle ranks, and thus make his relationship with Helena Justina respectable, Marcus Didius Falco has offered his services to Vespasian as a tax collector during the "great Census" of AD 73.

Whilst conducting the audit of two gladiatorial training schools, Falco stumbles upon the apparent murder of a star man-eating lion and an apparent rivalry between the schools.

When a gladiator also ends up dead, Falco takes on the investigation, one which leads him to Tripolitania.

They too have made their way to North Africa, drawn by Justinus' quest to find Silphium, an expensive herb already deemed extinct.