Dr. Edmund Hiedeman is brought to the hospital after a heart attack. The crotchety older physician can't resist giving orders to his attending physician, much to the dismay of his long-time nurse, Sharon Martin. Before she can intervene, Hiedeman goes to his lab and checks on his colleague and research partner, Dr. Barry Mayfield. He's unaware that Mayfield is reading a telegraph from a Dr. Brechman in Germany, advising more tests.
When Hiedeman comes in, Mayfield tells him that their transplant anti-rejection drug tests are going fine, and pressures Hiedeman to release their findings before someone else goes public first. Hiedeman says that they can wait to conduct further tests. Mayfield seemingly agrees and Sharon comes in to collect Hiedeman. Mayfield orders him to go to bed and tells him that he'll be conducting the valve operation for Hiedeman later. As Sharon goes, Mayfield reseals the telegraph envelope and tells her to give it to Hiedeman. As she leaves, another doctor mentions that he gave it to Mayfield some time ago, and Sharon realizes that Mayfield knows what it says.
Sharon gives Hiedeman the wire and warns him that Mayfield is a ruthless opportunist who is taking half-credit for Hiedeman's work despite the fact that he only started working on the project recently. Hiedeman insists that Mayfield is a talented professional and forbids Sharon from saying anything him.
Mayfield performs the valve operation and is forced to chastise Sharon several times when she is too busy watching him to do her job. Afterward, Sharon apologizes and admits she was worried about Hiedeman. As she cleans up, she notices some black suture on the floor and examines it. As she does, Mayfield secretly watches her in the mirror as she feels it. Sharon goes to her office and starts to call someone, but discovers the line is busy. Mayfield comes in and she asks him about the suture, saying that she didn't get it out during the operation and it doesn't feel right. Sharon accuses him of doing something to Mayfield, and Mayfield denies it. He calmly tells her to take her supposed evidence to the police if she suspects foul play, but to do her job in the meantime. He apparently leaves, but secretly listens as Sharon makes her call and sets up an appointment for a meeting the next day, and then writes it on a notepad.
That night, Sharon goes out to her car and finds Mayfield waiting for her... with a tire iron. He clubs her to death and then takes her house keys, being careful not to leave any fingerprints. He then tears through her purse, making it look like someone searched it for something specific.
The next morning, Columbo arrives at the murder scene, worn out from a night tending to his sick wife. He asks for orange juice and starts eating a hard-boiled egg, leaving shell pieces on the scene. He then confirms that someone searched Sharon's body.
Mayfield watches Sharon's apartment. Once her roommate, fellow nurse Marcia Dalton, leaves for the day, he goes inside and ransacks the place. He then takes out the vials of morphine he brought with him and hides them beneath the bathroom sink. He then goes back to the office and makes a call about Sharon. When a colleague informs him that Sharon is dead, Mayfield fakes shock.
Columbo comes in and notices that Mayfield is coolly setting his clock at the same time he claims to be upset. After introducing himself, Columbo asks if Mayfield knew anything about Sharon's personal life. Mayfield claims not to know anything and goes to check on Hiedeman. As they talk, Columbo expresses admiration for the surgeon's concentration on setting the clock despite the fact that he's distraught, but Mayfield dismisses it as a reflexive action. Mayfield checks on Hiedeman and when the elder doctor asks if his condition will change in a few days, Mayfield assures him that it will.
After receiving word of the break-in, Columbo goes to Sharon's apartment and talks to Marcia. She insists that her roommate had no enemies and that there was nothing hidden in the apartment. The police find the hidden morphine vials and dust them for prints, but they're clean.
An exhausted Columbo goes to see Mayfield, who is hosting a party at his home. As Columbo gets some food from the buffet, Mayfield comes over and asks why he's there. Columbo tells him about the morphine and Mayfield admits that Sharon had access to the drug cabinet at the hospital. Columbo then asks him about the name "Mac," which Sharon wrote on her notepad, but Mayfield has no idea who Mac is. After eating too fast, Columbo asks for stomach medicine and Mayfield gives him some pills. As they talk, Columbo explains that something bothers him about the entire set-up: there were no fingerprints on Sharon's murder weapon or at the apartment. A desperate drug addict like the one who supposedly Sharon wouldn't be so careful.
When it's clear Columbo isn't buying his framing of Sharon, Mayfield calls Marcia and invites her for a walk on the beach. As they talk, Mayfield manages to prod her into remembering a veteran, Harry Alexander, that Sharon worked with. Mayfield suggests that she call the police and drops her off at her apartment. She finds Columbo waiting for her and they go up to her apartment. When Columbo asks about Sharon's personal life, Marcia suggests that he investigate Harry. Columbo finds that odd and confirms that Mayfield is the one who got her to think of Harry.
Columbo goes to the hospital to see Hiedeman and confirms that Sharon was upset on the day of his valve operation. Hiedeman admits that he and Sharon had a close working relationship, and assumes that Sharon was upset because he was having surgery. Mayfield arrives and angrily orders Columbo to stop upsetting his patients. Outside, Columbo asks Mayfield if he knows Harry, but Mayfield claims he barely heard of the man.
The police track down Harry and Columbo goes to see him at the children's park where he works. Harry insists that he's clean, and that he hasn't seen Sharon in six months. He admits that he and Sharon were together for a while, but they broke up when she insisted that he was using her as a psychological crutch after he had dealt with his addiction. Harry asks Columbo not to say anything to his boss, because his history as a drug addict would get him fired. Columbo asks him if he knows who a Mac is, and Harry says he doesn't know of any such man.
Mayfield goes to his lab and finds Columbo talking with a cleaning lady as he goes through the garbage. Columbo explains that he's trying to track down the elusive Mac, and figures that the man is a key part of the murder mystery. Mayfield asks if he's arrested Harry, but Columbo says that he believes the man is innocent and that Sharon wasn't involved with drugs. He figures that someone planted the morphine to make it appear that Sharon was murdered by a drug addict, and notes that Mayfield and Hiedeman were the only other people who had access to the drug cabinet. Mayfield resents the implication, and Columbo insists that he doesn't suspect Mayfield. However, Columbo does point out that the cleaning lady saw Sharon after the successful operation and she was still upset, and he wonders why the nurse would have been upset once Hiedeman was out of danger.
Mayfield takes the next step to make it appear that Harry was the murderer, by going to the man's apartment. When Harry comes home, Mayfield drugs him unconscious and then injects an overdose of morphine into his left arm. Mayfield leaves and Harry wakes up, staggers outside, and falls to his death.
The next day, Columbo sees Mayfield and tells him about Harry's death. Mayfield figures that closes the case, but Columbo notes that Harry was left-handed and that he couldn't have injected himself in the left arm. The lieutenant figures that Harry's death is a set-up by the real killer. Mayfield suggests that Marcia knows more than what she's saying and attempts to implicate her, but Columbo doesn't believe it. The doctor jokingly notes that he has no motive.
Columbo goes to see Hiedeman again. As they talk, Columbo examines the medical supplies in the room. He sees the name of a supply company, Marcus and Carlson, and realizes that MAC is their initials. He calls their office and confirms that Sharon had set up an appointment to meet with a chemist at their office the morning after her murder. Columbo then talks to the clerk in the hospital supply room and confirms that the only chemical supply they get from MAC is suture. The clerk explains that the hospital uses two types of suture, permanent and dissolving. The dissolving suture has a different feel and color. Columbo is interested to learn that if dissolving suture were used in a heart valve operation, the pressure would cause the sutures to give in a matter of days and the subsequent death would look like heart failure.
Columbo talks to Hiedeman again and suggests that he get a second opinion. Mayfield storms in and threatens to have the hospital board file a complaint against Columbo if he continues to cause trouble with Mayfield's patient. Columbo goes with Mayfield to his office and explains that he now believes that someone substituted dissolving suture for permanent suture. Mayfield notes that dissolving suture is white, and Columbo agrees, pointing out that if someone dyed it, it would be murder. Mayfield laughs at Columbo and suggests the lieutenant is just making up theories. Columbo angrily slams a carafe down on Mayfield's desk and warns him that if Hiedeman dies in the next few days, they'll conduct an autopsy to determine if dissolving suture was responsible. He tells Mayfield to take good care of Hiedeman and leaves.
With time running out before Hiedeman dies, Mayfield tampers with his partner's medication. When Hiedeman has another attack, Mayfield orders him back into surgery. As he conducts the operation, Mayfield notices Columbo watching from the observation theater. As soon as the operation is done, Columbo and the police storm into the operating room with a search warrant, restrict everyone, and begin searching the room. Mayfield loses his temper for the first time and tries to shove past Columbo, but the police force him to stay. A thorough search of the operating room confirms that there is no dissolving suture, and that no suture was left inside of Hiedeman's chest. Columbo and Mayfield go to Mayfield's office, and Columbo admits that he was wrong. He starts to go, and Mayfield breathes a sigh of relief. However, Columbo comes back in and congratulates the doctor, admitting that he almost had him fooled. Columbo then explains that Mayfield losing his temper was very uncharacteristic, and that he realizes they searched everything and everyone in the room with one exception... Columbo himself. The lieutenant reaches into his pocket and takes out the bundle of dissolving suture that Mayfield planted on him when he tried to leave the operating room.