After returning from the multiverse, Ella Finn's life is back to normal until the police start asking questions about her missing cousins. So Ella goes looking for the family who hates her just as people start seeing monsters everywhere – including a secret monster-hunting government agency called The Institute sent to Saltburn to interrogate Ella. She knows the monsters are really Elementals breaking through the barrier between worlds and sets out to solve the mystery of how they're getting to Earth – and to stop humans hurting them.
When Ella's parents and friends are drawn into The Institute's plans to use Elementals as weapons, while the public is attacking anything perceived as 'monstrous', Ella has to break the promise she made to herself never to bring Elementals into the human world again. With the Book of All Life, gods and monsters will walk the Earth once more.