At Bellvue Hospital, New York, an intern is shot in the head by an unknown killer. Inspector Gordon of the 9th Precinct finds no obvious leads but senses an undercurrent of mystery at the hospital; enter Detective Fred Rowan, whose medical background enables him to pose as an intern. Through wheels within wheels, Rowan finally penetrates to a secret, dirty racket...and nurse Ann Sebastian, whom he's been dating, may be mixed up in it.

Young intern Dr. Foster arrives at New York's Bellevue Hospital with an ambulance. Although ward nurse Ann Sebastian has left a message asking him to contact her, he goes outside for a cigarette. While he walks along the river, however, he is shot in the head. During the subsequent police investigation, Foster's roommate tells Inspector Gordon that Foster had seemed worried and jumpy. Gordon also questions Ann, who can add nothing. When Gordon is unable to find Foster's killer, he brings in Fred Rowan to work undercover. Using the name Fred Gilbert, Fred, who has some medical experience, poses as an intern. After strict questioning by the head of the hospital, Fred joins the other new interns on a tour given by Dr. Alex Connell, who advises them to depend on the ward nurses for support. He then introduces Fred to Ann, with whom he will be working. Fred is assigned to room with Steve Anderson, a resentful young doctor, who is in love with Kathy Hall, a student nurse. Over the next few days, Fred learns that many of the interns are bitter about their jobs and lives. They are badly paid and thus cannot save enough to buy a private practice. Pop Ware, the elevator operator, tells Fred that he often places racing bets for the doctors, and Fred learns that many of them are deeply in debt to Pop. One day, finding Anderson overwrought, Fred advises him to marry Kathy and try to find happiness, but Anderson dismisses his advice. Meanwhile, Fred befriends Ann. When Fred gets off duty, he encounters Anderson, who reveals that he is getting married and leaving medicine for good. Fred asks Anderson to wait until they can talk about his decision, but when he later returns to their room, he discovers Anderson dead. At first, Fred is held as a murder suspect, but he is cleared when Kathy shows the police a suicide note that had been slipped under her door. Fred now believes that Pop is the key to the mystery and decides to place some bets with him. As his debt increases, Pop pressures Fred to pay him off. When Fred replies that he has no money to meet his debts, Pop threatens him by hinting that he had Foster killed and drove Anderson to suicide. Pop then suggests that Fred steal some drugs. Eventually, Fred realizes that Ann has been stealing drugs by withholding them from patients after they are prescribed for pain. Fred approaches her with Pop's suggestion, and she devises a plan for supplying him with drugs. When Fred delivers them to Pop, he increases his demand. Soon, Fred discovers that Ann, not Pop, delivers the drugs to the purchaser, whose front is a toy store. To precipitate a reaction, Fred tells Pop he will no longer steal drugs. Pop then comes after him with a gun, and Fred kills him in self-defense. Fred then arrests Ann, who tells him she needed the money from the sale of the drugs to help support her sister's sick child.