Meet the number one (and smallest) detective, Monsieur Mustard. Dressed to the nines, he is a charming and very famous crime-solving mouse. Monsieur Mustard curls his whiskers just so, slips on his fancy suit, looks at the handsome front-page picture on the newspaper announcing his arrival in the city of Bath, and chuckles happily to himself. As soon as he leaves the house, he is mobbed by admirers, as usual. But just as he starts to sniff out his first mystery, everything seems to go wrong: his detective bag is stolen, he is knocked off his feet and his monocle shatters. It appears that not everyone is happy to have the world-famous detective in their city . . . With the help of new friends, can Monsieur Mustard crack the case before the villain catches him?