In the morning, despite all of the alarm clocks having rung, Gerry has not risen. He is found dead in his bed of a chloral overdose. Tow of the young men, Jimmy and Ronny, drive over to Gerry's stepsister, Loraine's, house to break the bad news. They return to Chimneys where they search Gerry's room and find only seven alarm clocks. The missing one is found in the bushes at Chimneys.
Lord Caterham retakes possession of Chimneys. His daughter, Lady Eileen "Bundle" Brent, puzzled about the crime, writes a letter to Bill Eversleigh. It turns out that Gerry died in her bedroom. In her writing desk she finds an unfinished letter from Gerry to Loraine. In it, he states: "Forget what I said about the Seven Dials business." She decides to visit Bill in London. On the way, a stranger jumps out of the bushes in front of her car. She gets out as the stranger collapses to the ground, muttering "Seven dials...Jimmy Thesiger." Bundle manages to get him in her car and take him to the doctor where he's pronounced dead-- not from Bundle's car but from a gunshot wound.
The dead man is identified as Ronny, one of the young men at the party. IN the meantime, George Lomax receives a warning letter from the Seven Dials district of London. Bundles gets Jimmy's address in London and goes to break the news to him. Loraine is also present. Jimmy and Loraine reveal Ronny could have had ties to the Mafia. Loraine says that she discovered a list of names and dates together with an adress in Seven Dials. The three wonder if the missing alarm clock, leaving seven alarm clocks, was a warning left by the killer.
Bundle discovers that the Seven Dials is a seedy nightclub. She begs Bill to take her there. At the club, she recognizes the doorman as the former footman at Chimneys. She questions the doorman who tells her that a Russian named Mosgorovsky offered him three times the pay to work at the nightclub. Mosgorovsky also supplied a replacement for the footman, a man named John Bauer. The doorman shows Bundle a secret room with seven chairs. Bundle hides in a closet and eavesdrops on a meeting among seven people, all with white hoods with slits and clock faces. One member of the group talks about the upcoming party at Wyvern Abbey where a German named Everhard will be there with a new invention.
Bundle later learns that the invention is one that makes wire as strong as steel. Everhard is about to present the invention to the British at the party. Bundle and Jimmy wrangle invitations to the party where the find Superintendent Battle disguised as a waiter. Jimmy goes to the library to check something out. Bundle is told to stay in her room, but sneaks out the window and down the trellis to be a part of the action. Looking for Jimmy she hears a scuffle on the balcony outside the library and two gunshots.
Battle runs to the library where he finds Jimmy wounded and unconscious. Sir Stanley runs to his room where he finds the invention formula missing. Sir Oswald Coote arrives saying he was on a walk and discovered the gun. The next morning Battle searches the crime scene and finds one set of footprints leading up to where the gun was discovered -- Sir Oswald Coote's. He also finds a charred glove with teeth marks in the fireplace. Jimmy gets closer to Lady Coote's and receives an invitation from Lady Coote to their new house in Letherbury where he hopes to investigate Sir Oswald Coote.
Jimmy rings up Bundle and Loraine and tells them to meet him and Bill at the Seven Dials Club. Jimmy arrives, having left Bill in the car. Bundle shows him the secret room. In the meantime, Bill is knocked unconscious in the car. Bundle goes looking for brandy for Bill and is also knocked out. Mogorsky takes them to a meeting of the Seven Dials where the identity of Number 7 is revealed; it's Superintendent Battle. The Seven Dials is a secret group of criminal catcher and included Gerry Wade and Ronny Devereux.
It turns out that the secret formulae thief was Jimmy Thesiger, along with his accomplice Loraine, both of whom have just been arrested. Ronny was killed when he got too close to the truth. HIs last words were a warning to the Seven Dials about Jimmy, not the other way around. Jimmy climbed on the ivy and threw the stolen formula down to Loraine. He staged a fight in the library where he shot himself in the right arm. His right arm being disabled, he had to use his teeth to pull off his glove which he tossed in the fireplace.
The Seven Dials Mystery circa 1929 courtesy https://www.amazon.com/Mystery-Christie-Mysteries-Collection-Paperback/dp/0062074164.