In science fiction, when talking about stories that go sideways in time, the most obvious thing that comes to mind is that common sci-fi device; the parallel world. Rather surprisingly, it took Doctor Who until the 1970 Third Doctor adventure Inferno to tackle that trope. Maybe in another universe, the show got to it sooner. Investigating the Inferno Project, a dangerous experiment aiming to dig beneath the Earth's crust, the Doctor is accidentally transported to a parallel universe very different to his own. In fact, just like all your favourite parallel worlds, everything is exactly the opposite - and evil. Great Britain is an Orwellian Republic and his friends, those loveable military sorts at UNIT, are the fascistic Republican Security Forces led by a version of the Brigadier who wears an eyepatch. He must be evil! For giving us not only the show's first attempt at a Doctor Who alternate timeline but also a fascinating parallel world so different from the Doctor's own, this landmark sideways episode of Doctor Who really caught alight. Stifles laugh.
