June, 1925. A rare peaceful evening is cut short when Mary Russell and Sherlock Holmes receive a frantic phone call from an old friend. Veronica's aunt, Lady Vivian Beaconsfield, has vanished following a supervised outing from Bethlem Royal Hospital. Her adult life has been largely spent in and out of asylums - so why disappear now? And why is there no trace of the nurse who accompanied her?

In their search for the missing women, Russell and Holmes follow the trail from the cold, harsh hospital wards through Europe to the ethereal beauty of Venice. Caught up in decadent soirees, the rising tide of fascism and the myth of a haunted madhouse, the pair will discover that there are secrets hidden in the lagoon - and that every Venetian wears a mask ...