Aunt Saffronia drops the Sinister-Winterbottoms off at the Sanguine Spa, which is possibly owned by vampires, and where they must continue to complete the tasks they have been set if they are to see their parents again.

Following the clues left behind, Theo and Alexander find themselves at Siren's Song Seaside Amusement Park, where they must work together to escape, stop their newfound nemesis Essa, save Wil and their friends, and find their parents.

Twins Theo and Alexander and their sister Wil are abruptly dropped off at their Aunt Saffronia's house, where they learn they must complete a series of tasks in order to see their parents again--and their first assignment is at a waterpark. Conoce a los hermanos Sinister-Winterbottom: a la valiente Theo, a su tímido gemelo, Alexander, y a su hermana mayor, Wil. Los tres están atrapados durante el verano con su tía Saffronia, que no sabe cada cuánto...