(PG)

Saoirse Ronan proves that out-acting Keira Knightley in Atonement was no fluke with a charming performance here. She plays Lina, the teenage heroine of a fantasy set in the eponymous post-apocalyptic underground city. Ember is facing disaster as the massive generator that powers it starts to fail. Lina and her friend Don (Harry Treadaway) set out to save the city, outmanoeuvring corrupt city officials and other villains.

Ronan and Treadaway are heroes to cheer for, Martin Landau is splendid as Sol, the engineer who helps Lina, and Bill Murray is suitably raddled as the venal mayor in a thoroughly entertaining family adventure.

Gil Kenan's direction makes the most of the extraordinary otherworld sets and splendid special effects, and stages a thrilling climactic journey at high speed through underground caverns.