After thwarting a notorious villain at an eco-summit in Paris, the City Spies, a group of young agents working for the British Secret Intelligence Service's MI6, are ready for their next mission. One of the agents, fourteen-year-old Sydney, is a surfer from Bondi Beach, Australia, and she is assigned to go undercover on the marine research vessel the "Sylvia Earle." But things start to go wrong, and Sydney finds herself in hot water. Then, Sydney, Brooklyn, Kat, Paris, and Rio head to San Francisco, California, in pursuit of a lead that points to a potential mole in the organization.