Lionel Twain sends invitations to master detectives Dick and Dora Charleston, Inspector Sidney Wang, Milo Perrier, Sam Diamond, and Jessica Marbles. The invitation reads, “You are cordially invited to Dinner and a Murder at 22 Lola Lane, Saturday evening 7 P.M.” On Saturday night, the Charlestons are lost in the fog. Dick tries to call his host, but Twain’s telephone line has been cut by an unseen culprit. Sidney Wang and his adopted Japanese son, Willie, stop to read a map and encounter Dick. In another car, Tess Skeffington recites Twain’s dossier to her employer/paramour, Sam Diamond. The car runs out of gas and Tess is required to hike five miles to the nearest station. When Sidney and Willie arrive at Twain’s door, they barely miss being struck by a falling gargoyle. They are greeted by Bensonmum, the blind butler, who takes them to their room, where “a cozy fire” is consuming the bed. The Charlestons arrive next, sipping martinis to ease the shock of their close call with a falling gargoyle. Bensonmum leads them to their room, which is covered in dust and cobwebs. In the kitchen, Bensonmum informs Yetta, the kitchen maid, of her duties. His effort is wasted, as she is deaf, but he cannot see the note explaining her condition. Bensonmum returns to the living room to find Milo, who complains that Marcel has received a minor injury from a falling gargoyle. Sam is less passive about his gargoyle encounter and threatens Bensonmum at gunpoint. The guests gather in the drawing room for cocktails and deduce that the world’s greatest detectives have been brought together for a reason. Sam and Tess enter, followed soon after by Jessica Marbles and her wheelchair-bound nurse, Miss Withers. The group is summoned to the dining room. Bensonmum serves soup from an empty tureen. He apologizes and goes to the kitchen to fire Yetta, still unaware of the futility. After narrowly escaping death from poisoned wine and falling swords, the guests meet their host, whose entrance is accompanied by flashing colored lights. Twain announces that he will prove himself to be the world’s greatest criminologist. A murder is to be committed at midnight, and both the killer and the victim are currently at the table. Twain wagers $1 million against their reputations that none of the five detectives will be able to solve the crime. With the push of a button, he seals them inside the house, and then makes a dramatic exit. Yetta storms into the dining room and “screams,” although she produces no sound. Sidney, Jessica and Milo go to the kitchen, where they discover the body of Bensonmum, and notice a butcher knife missing from the knife rack. Locked out of the dining room, Milo retrieves the key from Bensonmum, but the body has disappeared and only the butler’s clothes remain. They find the dining room deserted, but Twain, watching from the moose head on the wall, assures the detectives that no one has left the dining room and instructs them to leave the room and re-enter. They follow Twain’s instructions and find all the guests in place. The sound of gunshots emanates from the kitchen, prompting Dick and Sam to investigate. Bensonmum’s body has reappeared, but without clothing. While Dick uses the bathroom, Sam returns to the dining room, which is again deserted. When he opens the bathroom door, Sam sees a cobweb-covered bed occupied by the Charlestons’ terrier. Dick reappears when Sam closes the door and reopens it. Sam enters the dining room and tells the other guests of his experience, which leads them to deduce that Twain has “two of everything” in the house. Milo leaves the room to demonstrate by knocking three times. Dick returns, followed moments later by Milo, who has been squeezed into Bensonmum’s suit. As the clock is about to strike midnight, Dora notices that Yetta has disappeared and there is a knock on the door. Twain stands in the doorway and falls forward, revealing a butcher knife and multiple stab wounds in his back. Dora concludes that Yetta is the culprit, but Sam discovers a case containing Yetta’s components, proving the maid to be an automaton. Dora then asserts that one of the guests must be the murderer. The guests assemble in the drawing room, where they posit their theories. Dick believes the murder was an elaborate suicide. Sidney suspects Dick, explaining that the Charlestons are destitute and have been borrowing money from Twain at seventeen percent interest. Dick draws attention to Jessica, explaining that Twain, her former fiancé, left her at the altar. Milo suggests patricide, and Tess admits that Twain was her uncle, but he was also Sidney’s adoptive father. When Twain realized, after nineteen years, that his son was Asian, Sidney was forced to leave home. Wang counters by reminding Milo of Marie Louise Cartier, Milo’s poodle, which Twain hunted and killed. Tess divulges that Sam also hated Twain because he had photographs of Sam wearing women’s clothing from the night they met in a gay bar, although Sam argues that he was undercover. When they retire to their bedrooms, the Wangs are threatened by a deadly snake, the Charlestons are frozen in fear as a scorpion crawls toward them, Jessica and Miss Withers smell poison gas, Sam and Tess receive notice of a hidden time bomb, and Milo and Marcel are about to be crushed by a descending ceiling. In the drawing room, Bensonmum crosses the detectives’ names off of a list. However, each has miraculously cheated death, and offers his or her solution in hopes of winning the $1 million wager. Sidney relies on the old adage, “the butler did it,” while Jessica knows Bensonmum to actually be Irving Goldman, Twain’s attorney. Dick suggests that Bensonmum is an accountant named Marvin Metzner, but Milo believes him to be Twain’s daughter, Rita, whose hatefulness stems from her manly appearance. Sam announces that he and Tess are actors, and Bensonmum is the real Sam Diamond, who created this ruse to prove his superiority to his colleagues. After humoring each of them, the butler removes his mask and reveals himself to be Twain. He berates the detectives for exploiting their readers with nonsensical surprise endings and other unscrupulous devices. Their reputations will be ruined when the world learns that Twain outsmarted them. As the guests leave, Sam admits to Tess that he really is Sam Diamond, while Dick explains to his wife that everything they’ve experienced was fake, including the fog. Inside the house, Twain removes another mask, exposing the face of Yetta, who laughs hysterically.