"The Thea Sisters are headed to Venice, Italy! They'll be there for the Carnival, an annual festival that's famouse for its elaborate masks. The mouselets love exploring the city's bridges and canals, and seeing the beautiful costumes mice wear to Carnival celebrations. But a mysterious thief strikes while they are there! Can they unmask him before the festival ends?"--Page 4 of cover.

The underwater kingdom of Aquamarina is in trouble because the enchanted Music of the Sea has stopped playing--so Thea and the Thea Sisters, together with their friend Will Mystery set out on a dangerous mission to find the Sea Violin which will restore color and life to the Kingdom.