It Happened Here is a 1966 documentary-style account of a Nazi occupation of Britain, made over the course of eight years of weekends by Kevin Brownlow and Andrew Mollo. They were 18 and 16, respectively, when they started production.
All the accounts of the film describe the production design as fanatically authentic, and praise the evocation of its 1940 setting through a combination of montage, attention to mundane detail, and damningly plausible British political accommodation of fascism.
The movie is now out on Region 2 DVD, and Brownlow's book, How It Happened Here, about the production and the controversy that swirled around the film, was reissued last year.
DVD's of the week: It Happened Here [telegraph.co.uk via metafilter]