This book constitutes the first full volume dedicated to an academic analysis of theatrically-released spinoff films derived from British radio and television sitcoms. Regularly maligned as the nadir of British film production and marginalised as a last resort for the financially-bereft industry during the 1970s, this study demonstrates that the sitcom spinoff film has instead been a persistent and important presence in British cinema from the 1940s to the present day, and includes (occasional) works with distinct artistic merit. Alongside an investigation of the economic imperative underpinning these productions, i.e. the exploitation of proven product with a ready-made audience, it is argued that, with a longevity stretching from Arthur Askey and his wartime Band Waggon (1940) to the crew of Kurupt FM and their recent People Just Do Nothing: Big in Japan (2021), the British sitcom spinoff can be interpreted as following a full generic ‘life cycle’. Starting with the ‘formative’ stage where works from Hi Gang! (1941) to I Only Arsked! (1958) establish the genre’s characteristics, the spinoff genre moves to its ‘classic’ stage where, secure for form and content, it enjoys considerable popular success with films like Till Death Us Do Part (1969), On the Buses (1971), The Likely Lads (1976) and Rising Damp (1980); the genre’s revival since the late-1990s reveals a more ‘parodic’ final stage, with films like The League of Gentlemen’s Apocalypse (2005) adopting a consciously self-reflective mode. It is also posited that the sitcom spinoff film is a viable source for social history, with the often-stereotypical re-presentations of characters and events an (often blatant) ideological metonym for the concerns of wider British society, notably in issues of class, race, gender and sexuality.