When an English cartographer arrives in Wales to tell the residents of the Welsh village of Ffynnon Garw that their “mountain” is only a hill, the offended community sets out to remedy the situation. The film is based on a story heard by Christopher Monger from his grandfather about the real village of Taff’s Well, in the old county of Glamorgan, and its neighbouring Garth Hill. However, due to 20th century urbanisation of the area, it was filmed in the more rural Llanrhaeadr-ym-Mochnant and Llansilin areas in Powys.
After breaking-up with his girlfriend Veronica, the unemployed John Hare rents a cheap room in an old boarding house owned by the nice Martin Stone and the landlord tells him that the house is crowded by discreet persons. John does not see any other tenant but a bizarre old woman in the house and during the nights, he sees weird things on his television and hears violent knocks on his door. When John calls Veronica, she notes that he is near a breakdown after many sleepless nights and decides to stay with him. However, Veronica vanishes during the night, leading John to an ultimate decision.