The title refers to a simple copper signet ring, with a portrait of Venus, worn by the suspect Severina Zotica.
The story begins in Rome during late spring, AD 71. Falco is in the Latumiae Prison, accused by Anacrites of having stolen imperial lead (Shadows in Bronze). Bailed out by his mother, Falco is heading across the city to visit Helena Justina when he is beaten up by his landlord's bullyboys for defaulting on his rent. When he finally arrives at the senator's house near the Capena Gate, he finds Helena in a reception room. Marcus decides to resume working as an independent, despite the fact that this means he is unlikely to be able to earn enough money to buy himself into a higher rank so that he can marry Helena.
Hyacinthus arrives at Falco's apartment in Fountain Court to inform him of possible clients. Falco agrees to visit the Hortensii, who live on the Pincian Hill. There, Sabina Pollia informs him that all of the Hortensii (Crepito, Felix, Novus and their wives) live together in the one house. Former slaves in the same household, they have now earned their freedom and set up in business together. Novus is the only one currently unwed and he is due to marry Severina Zotica. Sabina Pollia informs Falco that she believes Severina plans to murder Novus.
Falco investigates the claims. At the same time he also begins to hunt for a new apartment in which to live.
During the course of the investigation, Falco is once more arrested and imprisoned in the Latumiae. Two days later he is brought before Titus Caesar. Anacrites brings forth his charges regarded the lead. Falco is freed, provided he repays whatever is owed for the lead to the Emperor, and is asked to undertake more work for the Palace. He agrees to be available provided he is paid for previous missions that have already been completed. This is agreed to by Titus and he is invited to dinner. As a present, the Emperor's son promises him a turbot.
Returning to the house of the Hortensii, Falco discovers that Novus is dead on the privy after a banquet. He has been poisoned. Severina comes under suspicion. Falco continues to investigate, but can find no real lead. Sabina wants to pay him off and considers him a failure, when the news arrives that Viridovix is also dead. He too had been poisoned.
The arrival of the turbot becomes the cause of an impromptu party amongst friends and family of the Didii. Helena arrives, complete with baggage with which to move in with Falco, only to discover that she had not been invited to the party. Distraught, she attempts to leave and is prevented by Falco. Then Titus and the Praetorians arrive to sample the fish. At the end of the party, Helena remains behind.
Continuing to investigate the Hortensii, Falco is beaten badly by thugs working for Appius Priscillus. He is finally helped to Petronius', who sends for Helena. Helena takes him home to their new apartment.
When he finally recovers, he continues to investigate. He returns from the Hortensii in time to see the apartment block in which they are living collapse. Believing Helena to be inside, Falco and others begin to dig. Helena arrives just as all hope seems lost. Falco discovers that the Hortensii had owned the apartment block.
Falco and Helena move back into Fountain Court.