The Questers discover an alarming new threat. Without the adults, and the automatic cooling systems, the world's nuclear power plants are melting down and threaten to destroy the earth. The Questers have to get a message to the adults - but how? Callum arrives to collect the key to the Sparticle Project from the Ranchers, only to discover them homeless. Tia and Jeffrey don't trust Callum after past encounters. But the key has been lost and sold and Callum is the only hope to getting it back and to reuniting them with their friends.

• Jon East, Jonathan Fox Bassett, Tim Whitby

• Annette Badland
• Larissa Wilson
• Jerome Holder