The British government has a plan to drastically improve environmental conditions, thanks to the Gynt Foundation and its mysterious benefactor, 'Mr Hammond'. But the plan is bold and crucially requires the endorsement of a prominent green campaigner to allay any public fears. When the Doctor's former companion Polly sets off to investigate a fantastic, new, net zero city, nothing goes quite according to plan. For one thing, the Doctor himself unexpectedly appears. And for another, the city contains a terrible secret. With an older Dodo working for the government, the Doctor and Polly must fight a threat that endangers the whole of humanity.