12-year-old Ella Jones has been blind for two years and is navigating her new world with the help of her guide dog Maisie, her sister Poppy and her best friend Finn.

One day, on a visit to central London with her family, a light in the world is suddenly extinguished, causing panic and chaos as people are plunged into pitch black. A pagan god of light, Lugh, has been released from his millennium-old-prison by a shadowy villain and, out for revenge on humans, has brought darkness down on the world. But Ella is the hero the world needs: she is used to living her life in darkness every day, and is able to keep calm and work out a plan to defeat Lugh and return light to the world.