In a small town bank, bookkeeper Robert Sheldon (Lloyd Hughes) realizes that there is little opportunity for advancement. The cashier and his assistant are already elderly veterans of 20 years—and in his imagination he sees them two decades hence, older still, but ready to the last. Robert has ambition and dreams of becoming an executive in a New York office, prompting him to quit his job. His mother, Martha (Claire McDowell), gives him a letter to the one man in the city who can help.
In her room, she recalls "the everliving yesterday" in slight soft focus. Her husband, Willard Thatcher, (Joseph Kilgour) had returned from a long drunken debauch to accuse the (married) friend who had been helping her of fathering her child, and abandons them. Robert has no memories of Thatcher, and an intertitle reveals him to be a womanizer and businessman who operates just barely within the law. Although Martha just asks for their son the assistance he has a right to expect, not his father's name, Robert reminds Thatcher of his wife. Stifling paternal emotions, he resolves to avenge himself on her by sending Robert back, destroyed. The deviltry in his emotions is demonstrated as the camera moves steadily closer on his face, until flames superimpose over it.
Thatcher hires Robert on condition that he does as he is told; he reveals to his assistant that he plans to use the boy's honest face for his own benefit. Thatcher throws a nightclub party to convince Henry Godfrey (Andrew Arbuckle) to invest with him, and there Robert meets Dolly Wilson (Betty Ross Clarke), a showgirl whose heart remains pure although Thatcher obtained her a job. Robert and Dolly are soon in love, despite his refusal to believe her warning about Thatcher.
Robert is assigned to help put the deal through with Mrs. Godfrey (Edith Yorke), who is joining her husband at a dinner party thrown by Thatcher. Despite the threats, Robert's ethical qualms surface as he honestly tells Mrs. Godfrey that Thatcher's proposition was a gamble that would probably lose. Infuriated, Thatcher reveals the past, accusing Martha of infidelity and calls Robert the product of such a union. A struggle ensues, and when Thatcher pulls out a gun, it falls to the floor, and a bullet by chance strikes him. Thatcher tells his butler that Robert shot him, and his mistress, Fan (Betty Blythe), backs up the story as his dying wish, the final perversion of the fatherly emotion. Thatcher threatens Robert with the electric chair, shown in an intertitle.
During the hopeless trial, Robert refuses to give evidence, determined to keep his mother from involvement, and demands that Dolly not tell her either. Meanwhile, Fan is frightened by a fortune teller, for she knows the truth, and drowns her torment in alcohol. The prison scenes with Robert and Dolly are shown sparingly, with only the shadow of the cell bars and the guard's profile.
Desperate, only five days before his execution, Dolly goes to Martha and tells all. The pair go to Fan, who finally recants when Martha nearly strangles her with the fury of a mother's hatred. At this point high melodrama intrudes, as the background of a violent storm complements the emotions—a typical "Ince punch." The District Attorney (Andrew Robson) is unable to call the prison warden when a storm downs the phone lines. They drive through the downpour as Robert begins his death walk. Other prisoners are overcome seeing Robert approach death, and the shadow of the procession is glimpsed through prison windows. Three miles away, three minutes to midnight, the automobile is blocked by a passing train. Robert is strapped in the chair. Only by stopping in the power house on the outskirts of the city, cutting off the prison's electricity, is Robert's life saved with seconds to spare. He is led back to his cell, and the D.A. arrives to give the news to the warden.
The gripping story adds a realistic, slightly gruesome touch during the final scenes in its detailing of the electrocution apparatus, adding true suspense as to whether Robert will be saved in Mother o' Mine. The very ungoverned electricity that charges the atmosphere, filling the sky with lightning, is the same force that, disciplined by society, would have resulted in Robert's execution. The power must be pulled at its source, ultimately, bypassing the strict limits of the criminal justice system, to save his life.
Motion Picture News noted that "The last reel or so is undiluted melodrama building to a smashing climax that merits praise for director, 'scenarioist' and camera man. Few pictures have been able to show as much action and gripping screen drama for a finish ...." To relieve the tension, an allegorical shot of Father Time with his scythe shows him visited by a baby in a ribbon loincloth and Western Union hat. He carries a telegram from Joe Stork that a baby has been delivered to the Sheldons, 18 months later. Martha, Robert, and Dolly beam over the new addition to their family.