‘Look.’ She tilted the marionette’s body to the inquisitive flame. ‘It’s as if the Thames did the painting for me. And anyway, when people watch our shows, the dragon changes colour in their hearts. I think they mind-paint the dragon in the colours of their dreams.' Always on guard for Big Ben’s call that announces the dangerous Pea Soup fog is on its way, Wren spends her days searching the Thames foreshore for little treasures while her dad tries to earn a feeble living by performing in the Piazza of Covent Garden.

Their lives take a dangerous turn when Wren finds a driftwood figurehead that she carves into a dragon marionette. After becoming a crowd favourite, other jealous street performers leave her at the mercy of the lethal smog. She, however, isn’t killed by the Souper, but is transported to a magical London. The smog in this London is teeming with ash ravens and cloud ghouls while lights shine from London’s Eye to shield those who can afford to stay under the heavily taxed glow from the fog.

Wren, helped by her new friends and her unpredictable dragon—who tumbles to driftwood pieces at awkward moments—plunges into a life-or-death adventure. Their antics stretch from the water-filled London Underground where canal boats are pulled along by Were-mins to the Natural History Museum where Wren is helped by a Shaman to find her own magical light. While trying to find a way home, Wren needs to avoid the fog, flee from the government and help this other London re-light the dwindling flames of the Ferris Wheel before the fog engulfs the whole city.