Mma Ramotswe returns as the sole investigator for The No. 1 Ladies Detective Agency but her business is floundering financially. Her secretary Mma Makutsi is struggling to care for her sick brother as the business is not generating a profit.
They develop a flier to drum up business and a neighborhood boy named Wellingotn insists they hire him to distribute the pamphlet with good results. Three new cases come in - a disappearing dog, an absconding apostolic and a deranged dentist.
Petal Siphambe is worried that her husband, Peter, has run off with another woman and hires Mma Ramotswe to investigate his disappearance. She relates that he had recently become involved with apostolic church but she cannot identify the specific church. Mma Ramotswe questions several local religious leaders before finding the correct group. The pastor relates a miraculous disappearance midway through a river baptism. Mma Ramotswe returns Peter's abandoned clothing to Petal but she is dissatisfied and since his gold watch is missing still believes he may be philandering.
Mma Ramotswe is dubious about searching for Michael Mabizela's dog, Lucky, but accepts the case on the proviso that there will be a considerable fee. Wellington eventually solves the case himself when he finds the dog.
Mma Ramotswe's third case is more complex. She is hired by Jinger-May, a dentist's receptionist, to investigate Nigerian dentist Dr. Komoti. Jinger-May has noticed swinging moods and associated errors in the dentist's practice. Mma Ramotswe visits the dentist herself, despite a phobia of the profession, but finds him very capable. She continues to investigate with encouragement from Mma Makutsi and tails the dentist after work. He crosses into Nigeria and she is forced to break off but finds that he is quickly back in Botswana the next day.
Mma Ramotswe has a breakthrough in the missing husband case and uses Lucky as bait by the riverside. She lures a crocodile out of the water and kills it with her father's rifle before it reaches the dog. Upon investigation of the reptile's stomach contents she discovers Peter's watch. His widow later takes some comfort from learning that he was not unfaithful.
Mma Ramotswe once again tails the dentist but this time brings her own passport and follows him into Nigeria. She discovers that Dr. Komoti has a twin brother and flees when the men notice her spying on them. She realises that Dr. Komoti has been allowing his brother to pose as him and practice as a dentist using his qualifications. She decides exposing the doctor in Botswana would create a scandal that would put more people off of visiting a dentist and plans instead to force the doctor to leave.
She recruits her friends JLB Matekoni and BK to pose as police detectives and threatens the dentist with exposure. He is dubious but leaves the country nevertheless. However, Mma Ramotswe arranges for his arrest in Nigeria. She is satisfied that no scandal will occur in Botswana and the client Jinger-May promises her free dental care for life once she completes her training.