Bavaria, 1776. When Albrecht Durer the Much Much Younger's Frog Prints go missing, he knows exactly where to turn for help. Gretel (yes, that Gretel), now 35 and still living with her gluttonous brother Hans, is the country's most famous private investigator, and she leaps at the opportunity to travel to cosmopolitan Nuremberg to take on the case. But amid the hubbub of the city's annual sausage festival, Gretel struggles to find any clues that point toward the elusive thief. Even with the aid of the chatty mice living under her bed, the absent prints remain stubbornly out of view, and Gretel is forced to get creative in her search for the truth.

Gretel-yes, that Gretel-is now all grown up and working as a private investigator in 18th century Bavaria. Her professional interest is piqued when she begins to hear whispers of of mysterious goings-on off the coast of Schleswig-Holstein: sailors are disappearing, and there are rumors of mermaids and sea creatures and all manner of slippery, sea-based happenings. Ordinarily, Gretel's interest in sea-life does not extend beyond that which is edible, doused in butter and garlic, and already on the plate before her. However, funds are low, and the captain of the ship Arabella makes a tempting offer of good pay and a free cruise in return for her detective services. With a splendid new wig packed, Hans as her bodyguard on the journey north, and the promise of two weeks of fine dining and erudite company whilst sailing around the picturesque Friesian islands, what could possibly go wrong?