Tyrone Power in his farewell film plays Leonard Vole who befriends a dotty old widow played by Norma Varden. She even rewrites her will leaving him the bulk of a very large estate. When she's murdered, Scotland Yard arrests Power.
Power's solicitor Henry Daniell retains a dream team for defense of John Williams and the recently recovered Charles Laughton. Laughton is recovering from a heart attack and against medical advice plunges into the case. Laughton also has to deal with the efforts of his assigned nurse Elsa Lanchester to keep him following doctor's advice.
The original play this was taken from concentrated completely on the Power character and the machinations of his wife. Wilder built up the character of the nurse and barrister Sir Wilfred Robards so that they almost equaled the screen time of Mr. and Mrs. Vole. So much so that Charles Laughton was nominated for an Academy Award in 1957, but lost to Alec Guinness.
Marlene Dietrich plays Mrs. Vole. She's a war bride over from Germany and she's got her own agenda going. Her performance and what her character does is the key to the whole film. Dietrich probably would have gotten an Oscar nomination herself, but due to the fact that if her performance was hyped up for Academy consideration, the element of surprise would have been lost in the film. Wilder in fact apologized to Marlene for that.
The Anglo-Saxon legal system's goal is justice. Justice is served though not quite in the way it usually is in Witness for the Prosecution.
