In the opening scene of Episode 1 of Seven Dials, set in Ronda, Spain, in 1920, a man enters a bullring and discovers a card on the ground. Suddenly, a bull charges towards him, leaving him for dead. The story then skips forward to 1925 in Chimney, UK, where a lavish party is being hosted by Sir Oswald Coote and his wife at the residence of Lady Caterham and her daughter Eileen, affectionately known as "Bundle."
As Bundle makes her way through the room, she introduces us to the diverse cast of characters gathered at the party. We learn that Oswald Coote, a steel factory owner, is hoping to strike a deal with George Lomax, who works at the government's foreign office. The foreign office members include Gerry Wade, Ronny, Bill, and Jimmy. Ronny and Bill devise a plan to hide several alarm clocks in Gerry's room to prove that he can sleep through anything. Gerry is also a close family friend since he knew Bundle's brother, Thomas, who tragically passed away during the war.
Throughout the party, Gerry flirts with Bundle and asks her out for dinner on the following Tuesday, hinting that he intends to propose to her. Meanwhile, George Lomax orders Gerry to keep a close eye on Lady Coote and win her favor, as it would aid their upcoming business deal.
After an evening of dancing, Bundle retires to bed. In the morning, the alarm clocks start ringing as everyone gathers for breakfast. When Bundle goes up to check on Gerry, she finds him cold and motionless - he's dead. A doctor and detective are quickly called in. The doctor points to the sleeping draft bottle on Gerry's bedside table and suggests that it must have not mixed well with the alcohol from the party, leading to his death. It could be an accident or even suicide.
Bill acknowledges that Gerry was under immense work-related stress, but Bundle, deeply suspicious, counters that this cannot be true since Gerry had made plans with her for the upcoming week. This is also the new detective's maiden case, leaving him somewhat unprepared and at a loss. As she surveys the scene, Bundle observes Lady Coote arguing with her maid, Emily. The Cootes soon depart, introducing their secretary, a man named Bateman, who also attended school with Gerry and Tommy.
Returning to Gerry's room, the one where she met her demise, Bundle scrutinizes the scene for any clue. She notices the mark of the glass on the bedside table and finds a similar mark on the desk. In her anguish, she smashes the desk and discovers a half-written letter in which Gerry was apologizing to Lorraine, his sister, for mentioning the Seven Dials. Intrigued, Bundle enquires about the Seven Dials from her mother, who explains it's a district in London, not a particularly pleasant one. Bundle confesses her belief that Gerry was murdered and vows to uncover the truth.
Later, while wandering the garden, Bundle comes across a shattered alarm clock. When she questions Ronny about it, he insists they hadn't left any clocks on the lawn. He also mentions that they had left eight clocks in the room and hadn't placed any on the mantelpiece. This further convinces Bundle that someone was in Gerry's room, moved the clocks, and possibly threw one out. Ronny agrees to help her but asks her to keep it discreet.
Bundle speaks to Tredwell, who assures her that the staff had never noticed a bottle of medicinal draft on Gerry's table before this. She then confronts Emily and asks what Lady Coote said to her. Emily reveals that she had been assigned to Lady Coote, who was extremely demanding. This caused Emily to lose sleep, so Lady Coote gave her a sleeping draft bottle. However, the bottle disappeared from her room on the night of the party, and Emily has no idea how it ended up with Gerry.
On the day of the inquest, Gerry's death is ruled as a misadventure. Bundle, accompanied by Ronny, approaches Lady Coote and questions her about the sleeping draft bottle but is dismissed with a wave of her hand.
As Bundle notices a mustachioed man staring intently at her and Ronny, she follows him after he departs. She watches him huddle in a phone booth, making a call. When he exits, she approaches the booth and engages the operator in conversation, learning that the man had contacted Scotland Yard. With a swift goodbye to her mother, who laments her resemblance to their father, whom a picture reveals to be the same man from the initial scene with the bull, Bundle heads off to London.
Just as she's about to leave town, she encounters a startling sight: Ronny's lifeless body lying on the road, bleeding out from a gunshot wound. In his final breath, he urges her to warn Jimmy about Seven Dials before he passes away. The scene is both tragic and ominous, leaving an indelible mark on Bundle's journey.
