Mary-Kate and her mother are visiting Galinios, an idyllic Greek Island filled with history and surrounded by the shimmering Aegean Sea. An ancient mosaic has been unearthed at the local sardine processing plant and Professor Martin must investigate, leaving Mary-Kate to enjoy a few days of sunshine and antiquity.

But a message asking for help changes everything. A wrecked boat and smashed jetty have recently disrupted life on this tranquil island and point to a monster-sized mystery. Could the local legend of the Two-Headed Hydra be more than a story? If so, what could make this historically serene sea creature so angry?

Armed with her glitter pens and strawberry-scented notebook, Miss Mary-Kate Martin is determined to find answers. She might be scared of heights, but there is no problem too big for her to solve.