Harry S. Truman, played by Michael Ontkean, is Twin Peaks' sheriff who assists Special Agent Dale Cooper in the investigation of the murder of Laura Palmer. Harry is in love with fragile Josie Packard, and he is one of The Bookhouse Boys. Harry gets on well with Cooper. They hit it off almost from the start. Harry is very down-to-earth in contrast with Cooper's unconventional methods of policing, fascination with Tibet, dreams, etc. Harry regards Cooper as somewhat eccentric but well-meaning. In early episodes, Harry serves to introduce Cooper (and hence the viewer) to the more prominent residents of Twin Peaks. Harry begins to feel that he is the Dr. Watson to Cooper's Sherlock Holmes. Despite their differences, Truman represents a literary alternate to Cooper: they approach the same goal through different means. As the case progresses, Harry's respect for Cooper grows. He regards Coop as "the finest lawman he has ever known".
Harry shares his name with Harry S. Truman, the 33rd President of the United States.
Harry receives a call from Pete Martell, who tells him about the body he found on the shore behind his house, and later goes there to unwrap the plastic and take pictures of the scene. He is shocked to see that the corpse is none other than Laura Palmer.
He drives up to the Great Northern Hotel, where he tells Leland that Laura has been found dead, and then drives him to the morgue. Afterwards, he goes to the Twin Peaks High School and tells Bobby that Laura’s body has been found.
Sarah Palmer tells Harry about Laura’s activities the night before, saying that she remembers saying good night and that her phone rang once during the evening.
At the Calhoun Memorial Hospital, Harry meets with Dale Cooper, and escorts him to where Ronette is. They talk about the little connection Ronette and Laura had. On the way to the morgue, they bump into Dr. Jacoby, who tells them that Laura was seeing him.
After Cooper finds a piece of paper with the letter “R” on it under one of Laura’s fingernails, Harry asks him what is going on, and Dale proceeds to explain. At the sheriff department, Dale and Harry look through Laura’s possessions, including her diary. They found out that she was “nervous about meeting ‘J’” the night before. He also finds a key and some cocaine residue. Harry denies that it is cocaine, saying that Laura wasn’t that type of person. Cooper says that they’ll find out who she really was.
Harry and Cooper go to the train car where Laura was murdered, and find the other half of the necklace she shared with James. They also find a piece of paper with blood on it, spelling out “Fire Walk With Me”.
After going to the train car, Harry and Cooper go to the bank to open Laura’s safety deposit box, finding issues of Flesh World and ten thousand dollars in cash. In one of the magazines, they see a picture of Ronette Pulaski, discovering a connection.
At a town meeting held that evening, Harry tells Cooper about Josie and her husband, saying that he died in a boating accident the year before. He also tells him about the property rivalry with Ben Horne, and points out Margaret Lanterman, the Log Lady, to him. Afterwards, him and Cooper go to the Roadhouse to prepare a stakeout. They watch as Bobby arrives with Mike, and how Donna arrives and leaves with Joey Paulson. Harry pursues the motorcycle and eventually loses it, but later stops James and Donna, who are coming back from their meeting.
After coming back to the station, Cooper and Harry drink coffee at the station’s doughnut spread. Cooper says that he’s going to need a reasonably priced hotel room, and Harry tells him that he should stay at the Great Northern. He shakes hands with Harry and tells him that they will resume the investigation the next morning. Harry leaves the station to see Josie at her house, where they hug and look out to where Laura’s body was found.