The subject of the film is a man made island two miles from the shore on the east coast of England. Built in the 1970s the Outer Trial Bank is an experimental island, the purpose of which was to see if it were possible to build a larger barrage across the Wash and transform it into a fresh water reservoir. Further development of the project was abandoned when seawater was found to permeate the walls of the island. Tidal Island is a 12-hour time lapse of water entering and leaving the island.