Sent to Sing Sing prison, influential crook Spencer Tracy is unregenerate and refuses to adhere to the rules. While in solitary confinement, Tracy reconsiders his attitude. Thanks to the correctional facility's compassionate warden (Arthur Byron), Tracy becomes a model prisoner, even refusing to participate in a jailbreak. The warden sets up a special program permitting selected prisoners a degree of freedom and even suggests allowing an occasional furlough. When Tracy's girl friend (Bette Davis) is hurt in an auto accident, he is given a 24 hour pass to visit her. It's a test case--if Tracy doesn't return, the warden will be discredited and replaced. While on the "outside," Tracy learns that his old rival (Louis Calhern) was responsible for his girl's injuries. Davis shoots the rival, who in turn fingers Tracy as the one responsible; the convict thus risks execution upon returning to the arms of the law. Based on the book by real-life Sing Sing warden Lewis E. Lawes, 20,000 Years in Sing Sing was remade in 1940 as Castle on the Hudson.