The Capone ""family"" loyalty is tested when Frankie Rio's younger brother blows his cool and kills a member of a rival gang.
Capone and his men are infuriated when police raid a party honoring Frankie Rio's mother's sixtieth birthday. The cops later apologize, explaining that they were given an anonymous tip that the party was actually the site of a secret arms deal.
When the capos learn that rival gangster Aldo Genna provided that tip, Frankie's hot-headed younger brother, Johnnie, tracks Aldo to a restaurant and guns him down, killing an innocent waiter and wounding AIdo's brother, Mike Genna, in the process. The only person to see Johnnie's face is a restaurant patron, Professor Milton Revis, who rushes out in fear for his life.
