Season 7 comes to a close with Inferno. Project Inferno is a massive drill run by a massive tool to unleash massive energy from within the Earth’s core. The Doctor says it’s a bad idea, especially after some green goo starts turning the project personnel into monkey-zombies.
After futzing around with the TARDIS console in an attempt to leave Earth, the Doctor finds himself in a parallel universe where Britain is under the thumb of a fascist regime and Project Inferno is nearly complete. Now he must find a way to stop the drilling before everyone in both dimension dies in a fire.
Also notable for featuring the only Mirror Universe in all of sci-fi to feature less facial hair than its normal counterpart.