The Secret of the Mansion is the first book in the Trixie Belden

Trixie Belden

Trixie Belden is the title character in a series of 'girl detective' mysteries written between 1948 and 1986. The first six books were written by Julie Campbell Tatham, who also wrote the Ginny Gordon series, then continued by various in-house writers from Western Publishing under the pseudonym...

 series by Julie Campbell Tatham

Julie Campbell Tatham

Julie Campbell Tatham was a US Children's novels writer, who also wrote for adults, especially on Christian Science. As Julie Campbell was the creator of the Trixie Belden series and the Ginny Gordon series...

 under the name Julie Campbell.

Plot summary

The book opens with thirteen-year-old Trixie begging her mother for a horse. Her two older brothers are away at camp for the summer and Trixie is afraid she will be bored alone at their house, Crabapple Farm. The family cannot afford a horse, and Trixie will have to save up her money on her own, so imagine her delight when Trixie sees a horse trailer outside a neighboring mansion. The Wheelers, a rich family from New York City

New York City

New York is the most populous city in the United States and the center of the New York Metropolitan Area, one of the most populous metropolitan areas in the world. New York exerts a significant impact upon global commerce, finance, media, art, fashion, research, technology, education, and...

, have just bought the mansion, and are moving in with their teenaged daughter, Honey. Although tough Trixie thinks Honey is a priss and a sissy at first, they soon become fast friends. Trixie realizes that Honey really is a "poor little rich girl", her parents are never home, she grew up at boarding schools, camps and with governesses, and has been very sickly. Trixie of course helps her new friend overcome these things, and Honey balances Trixie's rash and impatient personality with her tactfulness and gentleness.

Recently, Trixie's neighbor on the other side was taken to the hospital, unconscious and sick from pneumonia

Pneumonia

Pneumonia is an inflammatory condition of the lung—especially affecting the microscopic air sacs —associated with fever, chest symptoms, and a lack of air space on a chest X-ray. Pneumonia is typically caused by an infection but there are a number of other causes...

. Trixie uses this chance to explore around the miser's rundown old mansion. Under the pretense of locking a window, Trixie and Honey climb inside to look around. They are shocked to find a tall, redheaded boy asleep on a mattress. The boy is Jim Frayne, the miser's grandnephew, and he is running away from his abusive stepfather. The three become close friends, with the girls smuggling food up to the mansion for Jim. Local legend says the old man hid his fortune somewhere in the house, electing to live in poverty after his beloved wife’s death. Honey and Jim are doubtful, but Trixie is sure the money is there somewhere, if only they could find it before Jim's evil stepfather comes looking for him.

Eventually, Jonesy, the stepfather, does come looking for him after a newspaper article about a jet crashing near the Miser's Mansion also features the miser's legend and an open safe Trixie accidentally leaves open when she finds the engagement ring that Jim's granduncle gave his wife.

That same night after Jonesy shows up, the mansion catches fire and is burned to the ground but when Trixie and Honey go there to the next morning to visit Jim, they find him gone.

He has run away again... this time in possession of a portion of the half million dollars. The money had been hidden in the mattress Jim had been sleeping on all the time.

The girls are confronted by the old man's (the granduncle's) lawyer who asks them to track Jim down for him. He wanted to track Jim because Mr. Frayne his uncle had formed a trust for him and he wanted to give it to him along with another guardian that would be appointed for him. Mr Rainsford can only do this if Trixie and Honey Track Jim down.