It goes like this: Doon (Harry Treadaway) and Lina (Saiorse Ronan) are trying to escape Ember, a dingy, Dickensian city, concealed miles underground, where the electricity is about to conk out. Those familiar with Jeanne DuPrau’s children’s novels will know all this, and more; but those who aren’t are hurriedly dumped in Ember without really knowing the city, its history or its inhabitants. Obviously, though, you know those kids should leave. Fans can cheer them on through the glaringly obvious plot and enjoy brief glimpses of character actors paying the mortgage (Liz Smith, Mackenzie Crook, Marianne Jean-Baptiste and Toby Jones, to name the UK contingent alone). It makes the Potter franchise look like the Three Colours trilogy.

PG, 95 mins

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