S03:E17 - The Imposters

The explorers and Finn search a cave in hopes of finding a portal that can cross dimensions. But once inside, an ghostly visage appears that sends Finn running. Marguerite, Roxton, and Challenger are unable to escape the glowing apparition, and are replaced by three spirits escaped from a world of darkness. Should they remain on the Plateau in the explorers' places, they will usher in a wave of chaos and destruction upon the world. Finn must find a way to save her friends, or else risk the future of humankind.