Dr. Ralph Snyder (John Beal) and Dr. Frank Blake (Edward Norris) open an office together but soon split over a rivalry for nightclub singer Diana Wayne (Florence Rice) and a difference over ethics. In an effort to make some quick money and marry Diana, Blake becomes a retainer for gangster Joe Grant (Douglas Fowley)while the upright (and uptight)Sawyer (John Beal)becomes a medical examiner in the district attorney's office. Grant is involved in a murder and forces Blake to remove an identifying scar, thereby proving that all gangsters should keep a doctor on retainment. But Blake has a change of heart and shows up at Grant's trial, spills the beans and Grant is convicted. Consequently, Blake loses his license, Diana and then his life, proving that the "do-tell' doctor should have heeded the film title.

On the night before he is to graduate and become a full-fledged doctor, intern Ralph Sawyer operates on car accident victim Diana Wayne, when no doctor is available. Ralph is prevented from graduating until Diana, a nightclub singer, makes a full recovery, but soon after he opens a practice with his friends, Frank Blake and Peter Piper. As they struggle to build their practice, Ralph and Frank also compete for the affections of Diana, who choses the more ambitious Frank. Frank unwittingly becomes entangled with gangster Joe Grant and his cohort, Barney Millen, when he accepts some free medicine samples from them. Later, Grant and Millen attempt to intimidate druggist Philip Cooper into joining a druggist protective association, and in retaliation, Cooper shoots Millen in the arm. Frank reluctantly patches up Millen and agrees not to report the bullet wound, even though he is required to do so by law. After Ralph gets a job with the medical examiner's office, Millen is identified by Cooper and brought to trial. When Frank testifies that he treated Millen for acid burns rather than a gunshot wound, Millen is released. A suspicious Ralph questions Frank, who admits that their increased clientele is due to Grant's patronage, and that Millen did not really have acid burns. When Frank refuses to tell the truth to the district attorney, Ralph quits their practice. Frank and Peter's business improves thanks to Grant and they soon move to a high-priced office. On the six-month anniversary of Ralph's saving her life, Diana invites him and Frank to celebrate at the club where she works with her brother Tom. Meanwhile, Millen shoots Cooper as he and Grant are driving by his drugstore, and just before he dies, the druggist identifies Grant by the scar on his face. Determined to escape detection, Grant forces Frank to surgically remove the scar by threatening to reveal his aid to Millen. Frank completes the operation, but takes X-rays that detail the surgery. Diana walks into the outer office as Frank is finishing up and quickly realizes what he has done. She refuses to speak with Frank when he later comes to her apartment, and soon after, Grant is brought to trial. Because his scar is now gone, it appears as if he will be released until Frank sends Ralph the incriminating X-rays. The judge will not accept them as evidence, however, when Ralph refuses to identify the doctor who performed the surgery. Upon hearing the news, Frank goes to the court and testifies about the operation, thereby helping to convict Grant, but at the expense of his own career. Proud of Frank, Diana and Ralph congratulate him, but just after his testimony, Millen shoots and kills him. Millen is himself shot by the police, and later, Diana, Ralph and Peter toast the memory of their friend.