The action takes place in late August 1833 at a hedge-school in the townland of Baile Beag, an Irish-speaking community in County Donegal. In a nearby field camps a recently arrived detachment of the Royal Engineers, making the first Ordnance Survey. For the purposes of cartography, the local Gaelic place names have to be recorded and rendered into English.

In examining the effects of this operation on the lives of a small group, Brian Friel skillfully reveals the far-reaching personal and cultural effects of an action which is at first sight purely administrative.