Discovering that the turtles on Turtle Island have gone missing, the Thea Sisters spot environment-threatening drums of a strange oil and struggle to gather help before spotting a pair of turtles entering a mysterious underground cave.

When mysterious things begin happening at the waterfall deep in the forest, Colette, Nicky, Pamela, Paulina, and Violet use their investigative skills to find out whether Vissia de Vissen is up to something nefarious.