Returning to her bestial friends, Kylie's quest takes an unexpected turn when they become separated, plunging her into the unknown realms beyond the reformed village. Jeys, the kaiju-sized toad, and Chatter, the oversized cat, find themselves in a magical Grove far from their home by the mill, where they must overcome challenges and confront their fears to become guardians of the One Tree.

In this sequel to Big Beasts, Patrick Scott's Lost Beasts delves into the other side of adventure, exploring its pains, costs, and struggles. As Kylie struggles to deal with the expectations of being the same girl she was before her previous journey, she rushes to save her friends, only to be thrust into the dark land known as the Taint.

Jeys must confront the loss of her foot to the Taint, while Kylie grapples with the realization that she too has been infected by the Taint. Accompanied by her mother and the miller, Kylie must break free from the darkness to help her friends, while Jeys must accept her role in the greater story.