Neverwhere (1996)

S1E1 Door

After Richard Mayhew comes to the aid of an injured girl on the street, he unwittingly embarks on a harrowing adventure that leads him into a bizarre alternate realm beneath London. The girl is fleeing from two ruthless assassins who she believes have murdered her family. But what is it that makes her their target? Now that Richard is entangled in her plight, he must confront the question: will he ever be able to reclaim his ordinary life?