The film focuses on the 'Plain Clothes Men', a group of detectives dressed up as average citizens to catch criminals without being noticed. They are especially hated by the underworld due to their constant meeting, during which suspects are analyzed and interrogated extensively. Among the staff is Dan Coghlan (Lon Chaney), a police officer with flat feet and a tough disposition, who is unsatisfied with the lack of adventure. As he is about to quit his job, he is noticed about a croaked jeweler. When arriving there, he finds Skeeter Carlson (Wheeler Oakman), a crook who never gets busted for a crime due to a lack of evidence.
Dan decides to follow him, and after talking to Skeeter's low-life girlfriend Bessie (Mae Busch) without gaining any information, he prevents Skeeter from seducing Myrtle Sullivan (Anita Page), an innocent flapper who finds excitement in hanging out with crooks. Dan has lately assigned himself as Myrtle's care-taker, and he disapproves of her boyfriend Marty (Carroll Nye), a dapper gangster without a job. When Skeeter is out of town for two days, Dan graps this opportunity to manipulate Bessie. After convincing her that Skeeter will soon dump her for Myrtle, Bessie admits that he croaked the jeweler.
Without wasting any time, Dan sets out to bust Skeeter and his men, only to find out that one of them is Marty. Shortly after, Bessie's body is found, and Dan is convinced that Skeeter is responsible for her death, considering that she was going to testify against him. The case against Skeeter is dismissed by the court, and he immediately reveals his plans on murdering Marty. Dan overhears this conversation, and hurries to Marty for protection, only to catch him in the midst of a robbery. Even though he is able to turn him in, Dan orders the police to leave Marty alone and helps him to take his first, tentative steps on the straight path.
Before leaving town, Marty wants to meet Myrtle one more time and sends her a letter, but Skeeter reads it before she can. He forces himself up to her, but is disturbed by police raid. Before they open the door, Skeeter fires a shot through it - which croaks a cop - and gets away. Upon finding out that she will testify against him, Skeeter sets out to kill her. Meanwhile, Dan tells Myrtle against better judgment that he loves her and then proposes to her. Even though she is actually in love with Marty, Myrtle accepts, mostly out of gratitude for all that Dan has done for her.
Afterwards, Dan leaves to find Skeeter, and catches him and his men preparing for a get-away. It results in a giant shootout, during which several policemen and gangsters are killed. Skeeter's men give in after being attacked by tear bombs, but Skeeter finds a way to escape to the rooftop. Dan follows him there, and after another shootout, Skeeter is killed. Meanwhile, Marty returns to town in rage after finding out about Dan and Myrtle's engagement. He proposes to Myrtle, but she decides to stay loyal to Dan. Dan realizes that she loves Marty, and allows them to be together.Lon Chaney as Daniel Aloysius 'Dan' Coghlan
Anita Page as Myrtle Sullivan
Carroll Nye as Marty
Wheeler Oakman as 'Mile-Away' Skeeter Carlson
Mae Busch as Bessie
Polly Moran as Mrs. McGinnis
Lydia Yeamans Titus as Mrs. Sullivan
William Orlamond as Dwiggins
Richard Carle as Wally
Chaney personally chose Anita Page as the leading lady, after seeing the rushes for Our Dancing Daughters (1928), in which she co-starred. By that time, Page was preparing for Bellamy Trial (1929), but she was reassigned to appear in While the City Sleeps. About working with Chaney, Page told in a 2007 interview:
"Before filming began, Lon talked to me about make-up and explained the action scenes to me. Finally, he gave me one last bit of advice: ‘Never act purely on impulse in important matters,’ he said. ‘Think things over carefully. Then, when you’re sure that you’re right, go ahead. And don’t let anything swerve you from that decision.’"
Los Angeles City Hall, which opened in April 1928, appears on film for perhaps the first time in the background of a few rooftop scenes.
All extant prints are missing portions from reels 6 and 7, and nitrate decomposition has affected portions of the surviving reels.
The original soundtrack for the feature is lost.