When an ancient bottle is found washed up on the beach after a ferocious electrical storm, all the residents of Eerie-on-Sea seem to want it ... but should they in fact fear it? Legend has it that the bottle contains an extraordinary secret that spells doom for the whole of Eerie-on-Sea. Could it be true that the vast sea creature Gargantis has awoken from her slumbers, and is causing the large cracks that are appearing all across the town? Finding themselves entrusted with the bottle, Herbie and Violet discover they will need to ride the terrible storm and pacify the creature if they want to save Eerie-on-Sea from sliding into the ocean and being lost for ever.