Doctor Who: Inferno (1970) written by Don Houghton and directed by Douglas Camfield
Third Doctor Jon Pertwee is advising UNIT (United Nations Intelligence Taskforce) on a project to drill through the crust of the Earth. The project is headed by Dr. Stahlman, a headstrong scientist obsessed with tapping into the power resources deep within the Earth. The project is nicknamed "Inferno." The Doctor urges caution; Stahlman ignores him. His urging is justified when the TARDIS control panel throws the Doctor into a parallel dimension where Inferno is nearing completion and things are not turning out well at all.
The story is interesting and doesn't seem dragged out even though it lasts seven episodes (166 minutes). The alternate dimension has a fascist England with the UNIT people acting as government enforcers rather than benevolent peacekeepers. The twist is fun and I'm sure the actors enjoyed a chance to do something different with their characters. Pertwee is his usual gruff self--a bit overbearing and a bit too serious. His comedy is slightly mean-spirited, making me like him less and less as I see more and more of him. I might be done watching episodes with him unless I hear or read something especially good about them.