June, 1925. Mary Russell and Sherlock Holmes are enjoying a well-needed relaxing summer evening in their home on the Sussex Downs. However, Russell soon receives a desperate telephone call from an old friend.

Veronica's aunt, Lady Vivian Beaconsfield, has disappeared following a supervised outing from Bethlem Royal Hospital. With Russell herself feeling less than balanced, the last thing she wants to deal with is the mad; however, she agrees to investigate. Having spent most of her adult life in and out of one asylum after another, Lady Beaconsfield at last seemed to be adjusting to her confinement at Bethlem.

So why did she disappear? 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 wards of the hospital through to the ethereal beauty of Venice. Caught up in decadent soirées, 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.