The Great Train Mystery — David Lewman

When Mr. Krabs gives SpongeBob the only key to the vault holding the Krabby Patty's secret recipe, SpongeBob vows to guard it with his life. But as soon as SpongeBob and Patrick board the train to retrieve the recipe the key is mysteriously missing. Who could have stolen the key? Was it sneaky Plankton? Or was it the “butler”? SpongeBob and Patrick are on the case in this Ready-to-Read adventure based on a special episode.