Betty Nelson, Mollie Billette, Grace Ford, and Amy Stonington form a Camping and Tramping Club and make plans for a hiking trip. While discussing their plans, Betty finds a five hundred dollar bill near a railroad line...

Grace Ford is thrown from her brother's horse, Prince, as she returns home with some important papers. The horse—and the papers—disappear! Even worse, Mollie's sister, Dodo, is trampled by the horse and may be permanently crippled. Mollie's family agonizes over Dodo while Grace's family worries about the lost papers...

As the Outdoor Girls ride in Mollie Billette's new car, Mollie nearly hits a girl who falls out of a tree, unconscious. The injured girl is taken to a nearby house but disappears before the doctor arrives. Mystified, the girls wonder who she is and why she ran away from them...

Mr. Ford must have someone stay at his lumber camp in the North Woods in order to prevent the theft of lumber. It is decided that Grace and her friends will stay in the camp until Mr. Ford is able to travel north. Complications develop when Alice Jallow's family claims to own some of the Ford land. Upon the girls' arrival, they discover that Alice and her family are staying on the adjoining Jallow property and have placed a guard on the disputed land. This development is most unpleasant for Amy, who has repeatedly been the victim of Alice's unkind remarks about her past.

After a foolish prank, Will Ford is sent away to work in his uncle's cotton mill in the south. After growing weary of his uncle's harsh treatment, Will runs away. Grace and her parents are frantic with worry, as Will has not communicated with them and has not returned home. Amy Blackford plans to journey to Florida with Mr. and Mrs. Stonington so that Mrs. Stonington can regain her health. Amy invites the other girls, and Betty and Mollie eagerly agree to make the trip. Grace is reluctant until a communication arrives from Will indicating that he is seeking his fortune in Florida. Grace and the other girls depart in high hopes of finding Will.

The Outdoor Girls and their friends vacation at Ocean View with Betty Nelson and her mother. After Amy Blackford learns that pirates sailed the waters near Ocean View, she becomes obsessed with the idea of finding pirate treasure. Amy is delighted when the girls dig up an old box that is buried on the beach. The box does indeed contain treasure—a fortune in diamonds! The box also contains a queer cipher, which the young people immediately attempt to decode. Their efforts prove fruitless, and they wonder whether the diamonds were smuggled into the country.

The Outdoor Girls plan to stay on Pine Island for the summer in a bungalow owned by Mollie Billette's aunt. Before the girls leave, thieves loot the town of Deepdale, stealing valuable family heirlooms from many people. Mollie is especially heartsick after her family's silver tea service is stolen. The authorities suspect a band of gypsies who recently left the area...

The Outdoor Girls do what they can for the young men fighting in the war in Europe. They work hard knitting sweaters and other necessary items. The girls are both proud and frightened when Allen, Frank, and Roy volunteer for the military. Grace is embarrassed and worried when her brother Will says nothing about volunteering and acts strange for weeks...

As the Outdoor Girls enjoy an afternoon ride, a motorcycle hits an old woman, leaving her lying injured in the road. The girls take Mrs. Sanderson to the Hostess House. After Mrs. Sanderson regains consciousness, she tells the girls how she has searched for her son for years and fears that he is dead. He ran away after being unjustly accused of a crime. The girls dread the boys' departure to fight in the war. What if they should be killed? The boys worry that the girls will find somebody else and are especially jealous of handsome Sergeant Mullins who has become friends with the girls.

The Outdoor Girls take a much-needed vacation from their duties at the Hostess House, but feel restless and plan to return. However, a fire destroys the Hostess House, so the girls must extend their vacation. The news that the boys are being sent to the front line in the war adds to the girls' feelings of despair. The girls welcome a trip to Bluff Point to stay in Grace's aunt's cottage. The girls are at first ecstatic in their new surroundings, but troublesome news arrives from home and abroad.