The title of the film refers to the blue lamp which traditionally hung outside British police stations.
The story, written by veteran T.E.B. Clarke (an ex-policeman), is on the cusp of the change to the "social realism" films that would emerge in that decade, but still holds close to a simple moral structure. The police are the honest guardians of a decent society, controlling the disorganized crime of a few unruly youths.
Warning: wikipedia contains spoilers
Decent old copper George Dixon (Warner) takes a new recruit, Andy Mitchell (Hanley), under his aegis introducing him to the easy-going night beat. Dixon is a classic Ealing 'ordinary' hero but also achronistic, unprepared and unable to answer the violence of Tom Riley (Bogarde). Facing a desperate youth with a gun Dixon walks to his own death almost uncomprehending. Despite this the ending is another Ealing quirk - ordinary decent society, including 'professional' criminals, band together to catch the murderer at a greyhound track.