Geronimo Stilton, editor of the Rodent Gazette, learns about a famous painting that is missing, along with the artist who was restoring it. When he helps his detective friend, Hercule Poirat, solve the case — from an open window and dropped business card that point to a rich factory owner — Geronimo misses his Valentine's Day date with Petunia Pretty Paws. In the end, Petunia forgives Geronimo, and they celebrate his birthday.