Summary: 

The film was part of the BBC Performance series, and it was based on a stage production of The Deep Blue Sea at the Almeida Theatre. The play tells the story of the passion of an older woman for a younger lover (Colin Firth), for whom she has left her husband-a love that is ill-fated. Following a standard theatrical convention, it takes place in the period of one day in one setting-a flat in London. (Summary by Friends of Firth)

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Information courtesy of The Internet Movie Database.