[3] 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.

In 1917, during World War I, two English cartographers, the pompous George Garrad and his junior, Reginald Anson, arrive in the fictional Welsh village of Ffynnon Garw to measure its "mountain", but everyone in town is at church, it being a Sunday.

The Reverend Robert Jones, who, after initially opposing the scheme, grasps its symbolism in restoring the community's war-damaged self-esteem, conspires with Morgan to delay the cartographers' departure while they build an earth mound on top of the hill and make it high enough to be considered a mountain.

Meanwhile, the cartographers' car is sabotaged, with the town mechanic deliberately fumbling around and breaking a part, informing them that a replacement needs to be brought in from Cardiff.

During the works, Reverend Jones collapses and dies from exhaustion and old age, and the mourning villagers bury him on the mountain.

After the burial, the townspeople convince Anson to stay the night at the top of the mountain and measure it at the break of dawn because their train leaves early.

A mechanic is asked about a nondescript broken part he has removed from a car, and replies "Well I don't know the English word, but in Welsh we call it a be'chi'ngalw."

The movie has resulted in a stream of visitors climbing to the summit of The Garth, and the Pentyrch History Society and the local community council have erected a notice on the mountain to explain its real historical significance.