The film maximised 20th Century-Fox's "frozen funds": money earned in England by the studio which, by law, could only be spent there. Hence, the extensive location footage is genuinely shot in London, whilst the interiors are all Los Angeles-based.
The central plot-gimmick about the mysterious "Mr Evans" and its final revelation was taken from the "Mr Christopher" strand in The House on 92nd Street (1945), also directed by Henry Hathaway in 1945. This is incorrect; The original novel "The Disappearing Nursemaid" aka "Warrant for X" was written by Philip McDonald in 1938. The plot device is in that novel, so the movie's device could not have been taken from a movie filmed in 1945; more likely the reverse is true.
The plot of this movies bears a striking resemblance to Rear Window: a person with a particular skill and physical disability sets out to solve a perceived crime with the help of his girlfriend and trusted companion.