Mma Ramotswe's business, the No. 1 Ladies' Detective Agency, has clients but needs to cut costs and increase revenue from fees. To reduce costs, she and her fiancé Mr. J.L.B. Matekoni decide to move the agency to the garage, which has plenty of office space. The original office will in turn be let, to add income. Mma Makutsi, secretary, is given the title of assistant detective, with a rise in pay. Mr. J.L.B. Matekoni is behind on his paperwork, which Mma Makutsi can organize. He has been lethargic lately. Mma Ramotswe realizes he needs help, and sets out to help him. He will not agree to see the doctor, so Mma Ramotswe asks Mma Potokwane of the orphan farm to step in.
Mma Potokwane brings him to Dr. Moffat who diagnoses him as having depression, for which he steps back from his garage while medications begin to work. Mma Makutsi takes over management of the garage and the useless young apprentices, making the apprentices accountable for their work, and making rapid business decisions to make good on the garage's name, Speedy Motors. She shows her strong management skills from the first hour of taking over her role as acting manager. The young apprentices are impressed with her, and how she applies her detective skills to solving some of the auto problems that the apprentices cannot solve.
An important Government Man, never named, approaches Mma Ramotswe to investigate his sister-in-law, whom he suspects of attempting to poison his brother. Mma Makutsi devises a way to gain access to the family, so the case is accepted, despite Mma Ramotswe needing time for her fiancé and their foster children. Mma Ramotswe is invited to stay at the family farm, so she can meet the family and investigate. While she is away, Mma Makutsi gains a client for the detective agency who wants work done in three days. Mr. Pulani runs the beauty contests in Gaborone. The present contestfor Miss Beauty and Integrity has five finalists; one is disqualified for theft from a store. The final selection is in three days. He wants to know if there is one finalist who has integrity. He is already under pressure from his financial backers for scandals the year before. He promises a largefee, writing the check as soon as Mma Makutsi agrees to take the case.
She travels to the university campus where one contestant lives, under her guise as a news reporter come to interview each contestant. The girl reveals herself to be shallow, a "bad girl". One of the apprentices drives her to meet the girls; Mma Makutsi realizes that any girls he knows will not be suitable to win the contest. He knows three of the four girls from his bar visits. Mma Makutsi then proceeds to the home of the fourth girl. She proves to be beautiful and modest, and her goal is to attend the Botswana Secretarial College, the same as Mma Makutsi attended. Returning to the office, she reports to the client with confidence that she is the contestant who matches the title of the contest.
At the farm, Mma Ramotswe meets all the family of the Government Man and a few of the staff in the house. She joins the family for a lunch that includes a meat stew. She is poisoned by this meal, as are several others in the family. She recovers and sleeps, waking well before dawn. Walking about the grounds, she encounters the cook, who is starting the fire in the house boiler. He once had worked in Gaborone as an assistant chef, but really did not like the work. He met the Government Man, who suggested he go to the family farm to be the assistant manager, as that was the work he sought, care of the cattle. Arriving at the farm, the brother took him on as the chef based on his experience. The cook had no success in making his case for a different job, so he began cooking badly in hopes they would push him out of that job.
Mma Ramotswe decides not to prosecute the cook for the risks he had taken. Back in Gaborone she confronts the Government Man with all the misunderstandings and hurt feelings of each person in that family, the real poison of so many secrets and unexpressed feelings. All of this is out in the open now and the cook is put to a different job. She then goes to see her fiancé at the orphan farm, where he has been connecting with the wild boy, teaching him words, making him toys. The two reach a vantage point above Mochudi, to see how the rains change the landscape. He is getting better.