The story takes place at the fictional St Gregory Hotel in New Orleans, owned by Warren Trent (Melvyn Douglas).
The hotel is in financial trouble. Hotel manager Peter McDermott (Rod Taylor) involves himself in the overtures of two different buyers. He also takes a romantic interest in Jeanne Rochefort (Catherine Spaak), an upper-class French guest, and deals with a wide range of routine problems, including a faulty elevator.
Jeanne is the traveling companion of one of the buyers, Curtis O'Keefe (Kevin McCarthy). He intends to renovate and "modernize" the hotel, with conveyor belts carrying luggage automatically around the building as if it were some sort of modern airport terminal, and even presenting the customer's bill on a conveyor belt. That was O'Keefe's vision for a hotel of the future, but his immediate plans for the St. Gregory are different: He would remove the fountain in the center of the lobby and replace it with a circular news stand and bookstore; he would remove the comfortable lobby seating, forcing guests to go to a restaurant or lounge and spend money to sit; he would change the mezzanine promenade with rows of little shops; and he would convert the great suites into smaller guest rooms.
Among the guests at the hotel are the Duke and Duchess of Lanbourne (Michael Rennie and Merle Oberon), a wealthy couple hiding out after fleeing from an accident in their car. A hotel detective, Dupere (Richard Conte), attempts to blackmail the Duke and Duchess. The Duchess responds by asking Dupere to drive the car from the accident to Washington D.C. for $25,000, but he gets caught outside of the city.
Keycase (Karl Malden), a professional thief, is working the hotel using a range of techniques and some female accomplices. In the beginning of the film he picks up a discarded key found in an ashtray at the airport. During the course of the film he sneaks into hotel rooms and steals the guests' money, but now that they can buy things by credit card, he find that most of the guests don't carry as much cash as they used to.
Meanwhile, a black couple, Dr. Elmo Adams (Davis Roberts) and his wife, attempt to rent a room at the St. Gregory, having previously made a reservation. However, Trent tells the desk clerk filling in for McDermott (McDermott having taken Rochefort to his French Quarter apartment) not to allow them accommodation. The Adamses are denied their room, the couple then disappear only to be followed by a man with a camera. When McDermott finds out he berates Trent for doing something that would jeopardize the deal with the union. After tracking them down to another hotel, McDermott offers the couple their room back, but when he goes to pick them up, they have already left the hotel. After contacting the NAACP, they inform McDermott that they had not had anything planned for the St. Gregory in terms of allowing blacks to check into the hotel. The couple then winds up in a Washington newspaper, damaging both O'Keefe's deal and the alternate deal with the union, leaving only the option of selling the hotel to a buyer who plans to destroy it.
O'Keefe makes a final offer on the hotel and asks Trent, who brings McDermott along, to hear it. During the meeting, McDermott gets a call revealing that "Dr." Elmo Adams is not a doctor after all and actually works as an employee for an O'Keefe Hotel in Philadelphia. McDermott also reveals that O'Keefe tried to pay him off and hints of his affair with Rochefort. Hotel owner Trent decides to reject the unscrupulous O'Keefe's offer and sell the St. Gregory to the man who will demolish it.
Keycase's luck changes when he blithely talks himself out of one tough spot by grabbing an ordinary-looking attache case, which belongs to the Duke and Duchess. He gets to a room, calms his pounding heart, and uses one of his key collection to open the case to see what it contains. The case is filled with cash. Counting himself lucky, Keycase heads for the elevator to leave.
In the elevator, Keycase is joined by the Duke and other guests. The elevator stops between floors as the control relays and emergency brakes begin to fail. McDermott and his assistant manager take the adjacent elevator to the same level and transfer passengers through the roof. The Duke and Keycase are the last two in the failing car. Keycase refuses to leave his briefcase, which contains the stolen money. The Duke is able to wrestle the case away and help Keycase out of the car, but right then the brakes completely fail, sending the Duke to his death.
The Duchess tells police she was responsible for the auto accident, hoping to save her late husband's reputation. She also absolves the blackmailing hotel detective of guilt. The detectives, seeing through the ruse, decide not to press charges. McDermott rounds up the remaining guests, including Jeanne, and buys drinks on the house as a final toast to the St. Gregory.Rod Taylor as Peter McDermott
Catherine Spaak as Jeanne Rochefort
Karl Malden as Keycase Milne
Melvyn Douglas as Warren Trent
Richard Conte as Det. Dupere
Merle Oberon as the Duchess Caroline
Michael Rennie as the Duke of Lanbourne
Kevin McCarthy as Curtis O'Keefe
Carmen McRae as Christine
Alfred Ryder as Capt. Yolles
Roy Roberts as Bailey
Al Checco as Herbie Chandler
Sheila Bromley as Mrs. Grandin
Harry Hickox as Sam
William Lanteau as Mason
Ken Lynch as Joe Laswell
Clinton Sundberg as Lawrence Morgan
Tol Avery as Kilbrick
Davis Roberts as Dr. Elmo Adams