Lovejoy travels to Venice to investigate the murder of a friend. The dead friend had bought a painting Lovejoy believed was a fake. In Venice, he is drawn into an ambitious art swindle, and identical twins Caterina and Lavinia are behind the scam.

In Venice, Lovejoy gets a job working for Caterina and Lavinia, with a view to busting their scam, and meets a Scottish forger called 'Lucky' Luciano. Lovejoy and Lucky together unravel the mystery of the Island of Rats.