At the core of the series is a group of young children who decide to help Dan McCormack, an impoverished man, keep his land, including a deserted gold-mining town and a valley. They turn it into a holiday camp for children. But attempts to whisk away Old Dan's ownership are constantly being made by a malevolent developer, William Woppa, who dreams of gaining a fortune when a highway is built through the valley.
All the local dignitaries attend the official opening ceremony of Secret Valley. The city's first group of kids, who have come to spend the weekend, also arrive, including "Spider" McGlurk dressed up as a girl.
Spider contacts a local pilot, a young amateur who likes to be called "The Red Baron". Spider's scheme is simple: he aims to make Secret Valley uninhabitable by dropping the biggest stench bomb ever made.
The people of Bildara feel honored and intrigued by their last neighbors in Spain. Mr. Ortega, a Spaniard who owns a dressage and riding school, opposes his daughter having a relationship with the boys from Secret Valley. At the same time, the best of Ortega's horses disappears and all suspicion falls on the members of the valley.
Mr. Van Gelder is angry because children are breaking into his junkyard at night and spoiling things, the fault lies not with the guys from Secret Valley but "Spider" and his two buddies: Serpent and Sparrow.
Cribbins punishes "Spider" and his gang since they have not succeeded in driving Secret Valley kids away. He will not provide them with chocolate until they prove they're worthy.
William Whooper intends to destroy natural life in Secret Valley. Paula, a most unusual young naturalist and vegetarian, is camping in the valley.
Secret Valley boys have proposed to the town boys a night campout on the beach. Spider and his gang decide to spoil the party.
Miles and Toshio are working on a new electronic project, a metal detector as a part of their school homework. But the gadget doesn't work as they thought since instead of detecting things it magnetizes them.
A runaway kid is aided by Samantha.
Secret Valley kids have a new project, is an invention of Miles and Toshio, who are still trying to win the competition for energy alternatives to oil.
Spider McGlurk sprinkles flour over Secret Valley Camp and, for once, manages to escape in his minibike.
The beach and bay are an important part of Secret Valley activities. Kids are teaching guests about how anglers lay down fishing nets.
Beaver feels displaced and pushed aside because of his age and height.
Jan Van Gelder, owner of the Bildara junkyard, was an ace pilot during the war and has kept his small plane as a souvenir in a hangar by the runway. He is requesting a permission to the council to give airplane rides to children, but Cribbins enjoys refusing it with the excuse that he's too old to fly.
The local newspaper gives away its old machinery to Secret Valley. The kids decide to make their own weekly newspaper to express their views on the events of Bildara.
The cousin of "Spider", Pee Wee, comes over and tells him that he can beat the guys at Secret Valley.
Rosa Ortega's family has settled in Bildara, Carlos, the father, trains horses on his farm for exhibitions in other cities. There is only one small problem, Rosa's mother is homesick for her native Spain.
The Secret Valley boys find a small chest containing, among other things, a candy recipe that turns out to be excellent. The boys decide to make large quantities and sell them to raise funds.
The Secret Valley boys are astonished when "Spider" swears he has mended ways and decided to be a good boy.
Beaver (Lofty), the smallest boy, decides to run away and leaves a note saying that he goes into the bush, and his buddies grow concerned.
The deeds to Secret Valley are rescued by the children before a 5.30 pm deadline. They relay them through difficult terrain, using a truck, motorbike, horse, grass skis (plus helmet), kayak, flying fox, a boat with an outboard motor, by foot in the nick of time.
The children from Secret Valley help to arrest a bank-robbing couple.
Children of the Secret Valley mount a rescue operation of a certain tree, whose leaves are eaten by koalas.
In the course of a battle between the boys of Secret Valley and the gang of "Spider", old Dan joyfully announces that Rosa has won a major writing prize.
Old Dan tells the boys that in the days of the Gold Rush, his father was convinced there was a hidden lode in Secret Valley. Beaver (Lofty) remains very impressed and decides to find the vein.