It's the sweltering summer of 1930, and Londoners are enjoying the heatwave. The Doctor, Romana and K9 plan to take a rest after their recent adventures, but the TARDIS warns them of time pollution in the locality. What connects the isolated Sussex resort of Nutchurch with the secret society run by the eccentric Percy Closed? Why has millionaire Hepworth Stackhouse dismissed his staff and hired assassin Julia Orlostro? And what is the truth behind the infernal vapour known only as Zodaal?