The fate of the natural world lies in the hands of three teenagers. Captured by The Circle, Tabby is taken to their headquarters, Undersea. She learns about their ancient sisterhood, sworn to protect the planet, and that she is one of "the Chosen". In London, Hayden finds herself at the centre of a coming together of disparate climate change groups. Denzi is missing, and Hayden's path to finding him is laced with danger. People all around the world are demanding clean air and blue skies, and on the cusp of humanity making change for the good, Tabby, Hayden, and Denzi's paths draw closer together. But as old friends arrive to help, old enemies resurface. The Circle's endgame comes into focus and Tabby, Hayden, and Denzi must race to prevent the destruction ahead. Can they learn how to harness Tabby and the Chosen's power in time before the world is changed forever?