Burke poses as a jewel thief and infiltrates a criminal syndicate planning to put New York City to sleep with gas introduced into all the steam mains. The crooks, Albert Indigo, Tucson the Cowboy, Charlie ""The Arm"" Segal, and Ziggy White, then plan to loot the entire city. With the finances for the plan suppied by foreign interests, the syndicate also intends to kill a few important, connected people. While planning takes place at the Villa Ruposa, ""Mr. I"" undergoes a senate hearing, after which he vows vengeance on Senator Burrows. With the aid of co-spy Ursula Prince, Burke learns about the entire operation, but a beautiful crook named Silkie kills Ursula so she can have Burke all to herself. Burke ultimately triumphs and gets word to the police, who wear gas masks to round up the gang.