At the end of World War I, infamous safecracker Square Kelly returns home to San Francisco, weathered by the ravages of war. Accompanied by Jack Riley, the soldier with whom he served in the trenches, Kelly descends the ship's plank to dockside, where he is greeted by Jack's father, Lt. Riley of the San Francisco police department, and Jack's sister Nora. Kelly then goes home to his aging mother, who informs him that his brother Jim and the rest of his old gang are waiting for him at Tierney's Café, the criminals' headquarters. As his mother eagerly recounts the gang's plans, Kelly begins to experience doubts about his former way of life. Nevertheless, Kelly goes to the café, where he is welcomed by Tierney's daughter Rose, a victim of her father's abuse. As the gang tries to enlist Kelly in their future exploits, Jack discusses his friend's reformation with his father, then calls to invite him for dinner. At dinner that night, Lt. Riley offers Kelly a job on the police force and a chance to go straight. Upon returning home, Kelly is torn between the Lieutenant's offer and Jim's entreaties to join him in a burglary planned for later that night. After agreeing to meet Jim and the rest of the gang in ten minutes, Kelly reflects upon the horrors of war and determines to relinquish his life of crime. When Kelly informs his mother of his decision, she denounces him as a coward and throws him out of the house. Outside the café, Kelly tells a relieved Rose that he plans to go straight, prompting her to warn him that her father will seek revenge. After Kelly announces that he is quitting the gang, Tierney challenges him to a fight and Kelly soundly thrashes him. To retaliate, Tierney decides to pit Jim against his brother. With Jim as his accomplice, Tierney breaks into a home on Kelly's beat. When Kelly discovers them, Jim refuses to shoot at his brother and Tierney shoots Jim in the back, drops his gun, then flees. Feeling responsible for his brother's death, Kelly visits his grieving mother and, after showing her the gun that killed Jim, proclaims that he is no longer acting as a cop, but as a man seeking vengeance for his brother's murder. Finally forgiving her son, Mrs. Kelly identifies the gun as belonging to Tierney. At the tavern, Kelly confronts Tierney. After Tierney shoots and wounds Kelly, Kelly returns fire and kills him, and then declares that Tierney was resisting arrest. While convalescing with Rose at his side, Kelly is visited by his mother. Their reconciliation is made complete when Kelly informs her that he plans to marry Rose and she offers her blessings.