Thea Stilton, who is the sister of Geronimo Stilton and the special correspondent of Mouse Island’s favourite newspaper The Rodent Gazette, is visiting her friend who runs a chocolate factory in Ecuador. Thea is not alone on this sweet visit, for the other Thea sisters are accompanying her too.

Unfortunately and quite suddenly, the factory begins to malfunction. Some bitterness has crept into the sweet times. Could it be someone’s sabotage or is it just a minor glitch is the doubt caused in the mind of the factory owner. It is up to the Thea Sisters to get down to the bottom of all that bitterness and bring out the sweetness.