Anne Beddingfeld is the daughter of the Professor Beddingfeld, a renowned anthropologist. When he dies of double-pneumonia, Anne is freed of all responsibilities and sets out to find excitement and adventure.

In London, Anne finds the excitement and adventure she craved when a man smelling of mothballs falls off a train platform and is killed. Another man claiming to be a doctor pushes through to examine the man, but it’s too late; nothing can be done. As Anne watches, the doctor beats a hasty retreat through the crowd, dropping a slip of paper as he goes. Anne retrieves the paper, which smells of mothballs and reads: Kilmorden Castle 17.122.

Police discover the dead man has an order to view "Mill House" in his pocket. Strangely, a woman is found strangled inside Mill House the very next day. Anne is sure the incidents are connected and sets out to investigate. She (along with everyone else in London) learns from a local paper that the woman was closely followed into Mill House by a clean-shaven young man in a brown suit. The police assume this man was the woman’s murderer, and a manhunt for the "man in the brown suit" is begun.

Anne tackles her own clue, the slip of paper, and discovers that ships are sometimes called "Castle." She asks at Cooks, and learns that, yes, Kilmorden Castle is departing for South Africa on the 17th—the cost is exactly that of Anne’s inheritance. She buys a ticket.

On the ship with Anne are several notable people: Colonel Johnny Race, Mrs. Suzanne Blair, Sir Eustace Pedler and his two secretaries (one amazingly sinister-looking, the other amazingly reclusive), and Reverend Chichester. After a few attempted murders and other strange happenings, Anne realizes that she is up against a gang of criminals led by a mysterious person referred to as "the Colonel." The key to the identity of the "Colonel" and the mystery of "the man in the brown suit" must be found among her fellow passengers.

Through luck, pluckiness, and determination, Anne slowly unravels a story that revolves around diamonds and leads her through South Africa—Victoria Falls, Kimberley, and Johannesburg. By the end, Anne finds not only the "Colonel" and "the man in the brown suit," but also love and friendship.

Spoilers ahead! Scroll down for full spoilers of this book.

The arch-criminal of the book is actually the amiable and amusing Sir Eustace Pedler.

The crime began with two young men: John Eardsley (son of a wealthy South African mine owner) and Harry Lucas. These men go to British Guyana to find adventure, and they discover carbon beds with amazing diamonds. They return to South Africa with a sampling of these diamonds.

Eardsley and Lucas fall in love with a South African woman called Anita Grunberg. Anita was actually in league with the gang of criminals and substituted stolen DeBeers diamonds for the South American diamonds. However, she held back some of the South American diamonds, instead of turning them all over to her boss, the Colonel.

Eardsley and Lucas are blamed for the DeBeers robbery, but Eardsley’s father buys them off. Disgraced, the two young men joined the army and fought in the war; Eardsley was reported killed and Lucas was reported missing and presumed killed.

Meanwhile, Anita took on the new persona of Nadina, a ballet dancer. She puts into action her plan to blackmail the Colonel with the South American diamonds she held back. Her accomplice is her husband, who comes to England on the Kilmorden Castle, hiding the diamonds in the cabin Anita would later use. This was the same man that Anne witnessed falling onto the rails. He had been tailed from South Africa by Harry Lucas, who was trying to get back the South American diamonds and clear his name.

In Nadina’s meeting with the Colonel (Sir Eustace), he strangles her but fails to recover the diamonds. Harry Lucas, who has tailed Nadina to Mill House in Marlow, finds her dead body and leaves the scene—once again he has been framed by the Colonel, only this time for murder. Unfortunately for Sir Eustace, someone has recognized him in Marlow (when he was supposed to be in Nice, and thus safe from suspicion). Sir Eustace’s secretary, Guy Pagett, was in Marlow (when he was supposed to be safely out of the way in Italy) and saw Sir Eustace. Guy Pagett is more worried that Sir Eustace saw him, rather than vice versa, however.

Meanwhile, Harry induces Sir Eustace (who he suspects is the Colonel) to take him as a secretary, hoping to find a chance to snatch the diamonds on board the Kilmorden Castle. No one suspects that Suzanne Blair, who is moved into what would’ve been Nadina’s cabin, is in possession of the diamonds. She and Anne discover this fact and agree to become partners in the search for the Colonel and the "man in the brown suit."

When Anne finally wrenches Guy Pagett’s secret out of him (he has a wife and five children in Marlow, which is why he was in Marlow instead of Italy and happened to see Sir Eustace there), she knows who the Colonel is. She allows herself to fall into a "trap" set by Sir Eustace, and she and Harry manage to capture him for Colonel Race.

In a final twist, we discover that Harry Lucas is not Harry Lucas, but Sir John Eardsley.

I think this is my favorite Agatha Christie book. It’s written as a combination of Anne’s narrative and excerpts from Sir Eustace Pedler’s diary. Anne’s narrative is full of romance and curiosity—she is the type of person you can fully imagine following the slightest lead to the other side of the world. Sir Eustace’s diary, on the other hand, is insightful and humorous. Overall the story is enjoyable to read on many levels.

As far as the mystery goes, it seems fairly typical of some of Christie’s early novels—a gang of criminals, spies from the war, jewels, a beautiful and mysterious woman .... When the true criminal is revealed, you will realize you had all the clues you needed to solve the crime—even Sir Eustace in his diary gives you hints. The tiny twist at the end adds one more surprise.