The big twist ending is completely revealed in the brief and horrible summary IMDb gives this one. I hope you didn't read this!!
Dick Powell plays a tough but honest detective. He's in a real bind when there's a murder and it seems as if his girlfriend is either the killer or knows who the killer is! Much of the film is a battle of wits between the detective and his girl and just how honest and decent he is becomes very evident in the surprise ending.
As usual, Dick Powell was excellent in the lead. His starring roles in "Four Star Playhouse" were actually much better than most of his movies because it allowed him to be more realistic--less the singing pretty boy he was in the 1930s. Because of this, I do recommend you see the show. While it's far from his best from the series (the ending, though exciting is also hard to swallow), it still shows his excellent style and grace.