Grab your magnifying glass and get ready to solve a massive mystery! Geronimo Stilton loves cracking difficult cases, and this one might be his most exciting yet. When his clever sister Thea uncovers a secret hidden inside Mouse Island's most famous painting, the Mona Mousa, the two siblings must team up to crack the code. You will follow the trail of tricky clues right beneath the busy streets of New Mouse City. Down in the dark, twisting tunnels, an absolutely fabumouse discovery is waiting for you. But cracking the ultimate code will require all your brainpower and bravery. Will Geronimo and Thea uncover the truth before time runs out? Dive right into this funny, fast-paced chapter book packed with lively pictures, cool puzzles, and a thrilling treasure hunt that will keep you on the edge of your seat!