Barney Nolan (Edmond O'Brien) has been a cop for 16 years and this has left him a cynical and sometimes brutal man. He hasn't always been living that far down the dark streets--his current partner--Mark Brewster (John Agar) is someone he took under his wing when Brewster was a kid and kept from a life of crime.
But now he's starting to jump over to the dark side on a big time level. When the 1954 film noir Shield for Murder begins, he's learned that a bookie is carrying $25 grand in cash. He gets the bookie into an alley, shoots him, takes the cash, then claims the guy made a break for it while being arrested.
His fellow cops, including Brewster, believe him at first. But a couple of sleezy private eyes working for a local crime boss show up looking for the money. Other holes appear in Nolan's story, not the least of which is in the form of a deaf-mute man who saw the murder from his window.
It's too bad for the deaf-mute guy that the detective he encounters when he comes to the station is Nolan. He only saw the killer from behind, so doesn't realize he's handing a note explaining that he's a witness to that very killer. That leads to yet another tragedy.
Shield for Murder, co-directed by O'Brien and Howard V. Koch, is a great-looking film. It doesn't break any new ground in its noir-ish look, but it does a fine job of giving us that look and wrapping a strong story around it. Barney Nolan is a great character, with the script and O'Brien's performance giving us a vivid portrait of someone who was once one of a good man, but is now gradually crossing one moral event horizon after another.
The rest of the cast is largely first-rate as well. Claude Akins is one of the sleazy private eyes and his shootout against Nolan around a crowded public pool is superb. John Agar as Brewster plays his part well. Agar was nearly always stiff when he had a lead role in a film, but he was generally pretty good in supporting roles such as this one. Marla English, who had a short but respectable career appearing mostly in B-movies, brings a nice sense of tragedy to her role as Nolan's girl, while Carolyn Jones is wonderful in a small role as a bar-fly.
Here's the movie on YouTube, though if you have Amazon Prime, that print is the better one.