In an unassuming office in London there's a team that have a very important job. Their numbers are few but their job is big. They need to keep the U.K. safe from those things that 'don't exist' except they do. Ghosts, aliens, time slips, sea creatures etc are all real. "There are more things in heaven and earth, Horatio, than are dreamt of in your philosophy."
Department 49 was set up a long time ago to deal with these things, protect the public and keep them non the wiser to the issues and the dangers.
So a call the no doubt puzzled that metropolitan police seemed perfectly normal to them. That is that there had been a train accident. But no normal train accident. A Victorian train had collided with a modern train. But the Victorian train hadn't simply been out of a museum on a tour. It had suddenly appeared on the track and it was full of Victorians. The department are called and have to deal with the aftermath and try and find out why it happened. It takes them on many journeys they didn't envisage.