The episode begins with a man named Roger Stanford building a bomb in his lab.
Roger Stanford goes into an office, where he sprays all the workers with string confetti. Buckner's secretary, Miss Bishop, comes out and stops him and takes him into another office. She asks him if he developed some pictures, and he ignores the question. He then takes a cigar pack out of his uncle's coat when she isn't looking.
Roger goes into his Mr. Buckner's office. Mr. Buckner is the head of the company and is trying to get Roger to agree to selling the company. He is vehemently opposed to the idea. Buckner's chauffeur, Mr. Quincy, is there, too. Roger complains that Buckner married his aunt to get control of the company. Buckner says that Roger has cost the company a lot of money and trouble in the past, and enough is enough. Quincy then reads from a list of things he found out about Roger, such as a previous drug problem, Roger forging his aunt's signature, and a car theft arrest sheet. Buckner prepares to leave for a trip to Pinewild. He tells Roger that he has to sign a statement withdrawing opposition to the sale, then make a statement to the press in the morning. If Roger doesn't comply, Buckner says he will tell Roger's aunt about all the trouble he's been in. Roger agrees. When he leaves, he stops and talks to the secretary, Miss Bishop. He leans over her and drops the cigar pack he took from his uncle's bag under her desk.
In the parking lot, Roger asks the limo mechanic to take a look under his hood. While the mechanic is occupied, Roger switches the cigar box in the car with one he is carrying.
Roger meets with Miss Bishop later at a restaurant. Meanwhile, Buckner is on his way to his cabin in the middle of a thunderstorm. Buckner opens the box of cigars, which is on a timer. After a few minutes, the whole car explodes.
Roger goes home and gets a typewriter out of Quincy's rooms and puts it in his car. When he's on his way back into the house, Columbo comes around the corner. Roger claims he thought he saw the door open, so he came up to investigate. Columbo says that Roger's aunt wants to see him right away, so they go to her house.
At the house, Roger's aunt, Dory, tells Columbo that the employees at Stanford Chemical have been harassing them because they are upset about the plan to sell the company. Roger tries to reassure Dory, but then Columbo plays back a message that was left for Dory. Buckner called and there is a recording of him asking for cigars from his driver. He then tells Dory that he Roger will come to the house that night to talk to her about something. As he is talking, Columbo notices Roger looking at his watch. After the message, Columbo spills his drink, and Roger gets very upset. Dory is surprised that Roger is so upset because she knows he doesn't like his uncle, but Roger says he's been getting to like him better lately.
Columbo starts asking Roger about his family. Roger gets frustrated and tells Columbo that Dory would inherit all of Buckner's money. Columbo is about to leave, and he comments that he noticed Roger kept looking at his watch. After Columbo leaves, Dory asks Roger what he wanted to talk to her about. He tells her that he asked Buckner if he could move to the legal department, and he was going to be doing so on Monday.
Columbo goes to Pinewild, where Buckner's cabin is located. He meets an officer there, who tells him that they found a car bumper down the road a little. They soon find the bodies, too. The officer says that it looks like there was an explosion inside the car.
Everett Logan, the vice-president, comes over to be with Dory after she hears about Buckner's death, and she tells him about Roger's plan to move to the legal department. He is a bit surprised. He tells Dory he will take care of all the business things and not to worry about it.
Columbo goes to check out Roger's lab, and Roger discovers him there. Columbo asks Roger for some scientific information because he thinks there was an explosion in the car. Roger says that a lot of the employees at the plant have Ph.D.'s in chemistry and that many people were unhappy about Buckner's decision to sell. Columbo then tells Roger he has a lead in the case.
They go to see Miss Bishop, and Columbo asks her about the cigars that she sent with Buckner on his trip. She tells Columbo about finding the cigar case under her desk. Roger tells Miss Bishop that Columbo thinks that Buckner was killed by an exploding cigar. Roger asks Miss Bishop to write down a list of the people who were at the office that day, as well as anyone who might have had access to the cigars. She mentions Everett Logan and says he often had access to the cigars when they came in.
Columbo goes to see Everett, who says he has four boxes of cigars left. He calls in his secretary to see if she knows what happened to the other box of cigars. Columbo asks him to lock up the cabinet that holds the cigars so he can fingerprint it later.
Columbo and Roger ride across the plant in a golf cart. Columbo shows him a letter that he found in a trash can. It is a letter from Quincy, saying that Logan was trying to upset Roger and use him to manipulate his aunt into going along with the sale. Columbo says he couldn't find the typewriter that the letter was written on, and he thinks that maybe Quincy had another place where he did some of his work from. Roger mentions that one night he saw a piece of paper in Quincy's wallet that had the name Harry J. O'neill written on it.
Columbo leaves, and Roger rushes back to his car. As he is leaving, Miss Bishop stops him. She is very nervous, and she tells him she's afraid someone will find out about their relationship. He tells her he got rid of the pictures he took of her and not to worry.
Columbo returns to the crime scene, where the officer tells him that part of the gas tank was found and that it did explode. He says it's strange, too, because some pieces of the car are burned, some are smashed, and some are still intact. Columbo tells him it's been difficult for him, too, because the autopsy reports showed no sign of flammable products.
Roger goes to Quincy's other house, where he leaves the typewriter that he previously took from Quincy's house. He then waits in the house until two officers show up. He drops a lamp on purpose, and the officers cover both exits, eventually catching Roger as he tries to flee. They take him to his aunt's house, and she is upset. Columbo shows up, and Roger tells him he figured out where Quincy's hideaway was. He asks to speak to Columbo alone so he can explain. Then another detective named Farrell arrives and says he found the typewriter, along with some files. Dory says she found some damaging information about Everett Logan in the pocket of one of her husband's suits. Columbo takes Farrell aside, and Farrell shows him a picture he found on Roger when he caught him. It's a picture of Miss Bishop with Mr. Buckner. Columbo starts to take Roger into another room, but Dory gets upset when she sees the picture. She says she doesn't want anyone else to know about the picture, then she throws Columbo out.
As he's leaving, Columbo overhears Roger talking to Dory. He tells her he knew about Mr. Buckner and Miss Bishop for years, and he tried to make it stop but couldn't. She says she's been a fool, and he hugs her. As he hugs her, he sees Columbo standing there watching and listening.
The next day, Roger goes into Stanford Chemical, where Miss Bishop is very upset. She says she has been fired, but Roger tells her she's not being fired, just transferred. He goes into his new office - the office that used to be his uncle's. He takes a picture of his uncle that is on the desk and places it facedown. Columbo and Everett Logan arrive, and Everett tells Roger that he's been fired. Columbo picks up the picture of Buckner and remarks that it must have fallen over. He then tells Roger that they found something at the crime scene.
When they arrive, the officer at the scene gives Columbo a package. Columbo, Everett, and Roger head up the mountain to see Dory, who is staying at the cabin. On the way, Columbo shows them that the package contains the cigar box, and it is barely damaged. Everett asks if that means it wasn't a murder, but just an accident, and Columbo says he thinks so.
Everett says he's grateful because he was starting to feel that he might have been indirectly responsible for Buckner's death because he was opposed to the sale. He thought maybe someone stole the box of cigars to kill Buckner, thinking that he was doing Everett a favor. Columbo starts talking about all the information that Quincy had gathered, and Roger gets upset. He keeps telling Columbo to stop talking about it, but Columbo continues. As he talks, he is careless with the box, thumping on it and tossing it around. Roger seems agitated by it.
Columbo starts telling Roger that he at first thought Roger was the culprit, and he had all kinds of ideas about how he could have done it. He then starts opening the cigar box, but Roger tries to stop him. Eventually, Columbo opens the box and offers Everett a cigar. Roger looks at his watch and becomes more and more nervous. Columbo comments about him looking at his watch again. Columbo continues talking about his theory about Roger. Roger gets very worked up and lunges for the box to throw it out the window. The cigars spill out all over the floor, and Roger scrambles to pick them all up. As he does so, Columbo reaches for one and lights it. He tells Everett he got them from his secretary. When Roger hears that, he realizes he's given himself away.