After becoming tired of a world full of grown-ups telling them what to do, Toughy, Smarty and Mouse decide to escape from their dreary city home in a converted lifeboat owned by the Elderly Boy.
After a long voyage, the lifeboat arrives at Grasshopper Island, though the Elderly Boy departs after deciding that the island is too lacking in comfort.
Having settled into an abandoned hut, the boys search for food. Smarty's fishing rod doesn't work, though Mouse has more success with snail-hunting. After a diet of snails, they found a rusty key that opened a hidden cupboard full of food.
The boys' staple diet of biscuits is running out. Mouse wants to find an egg tree whilst Smarty attempts to catch frogs. Eventually, success comes when they catch a chick- which then eats all the remaining biscuits.
After a month on Grasshopper Island, Mouse demands proof that it actually is an island. Meanwhile, we are introduced to the grasshopper expert Cornelius Button and his housekeeper Lupus, who are harvesting their apricots.
After 1.5 months on Grasshopper Island, Toughy, Smarty and Mouse search for buried treasure. Meanwhile, Lupus (who has an affliction to cheese), forbids Cornelius Button (who loves it) to keep it on the island.
Cornelius Button's sister threatens to visit her pupils because study has made them stupid. He writes a letter to his sister saying he now eats children. The boys overhear this and believe that the people in the Big House are cannibals.
Lupus has been visiting the boys. Cornelius misinterprets this, thinking she is ill and takes her away for a holiday. The boys try making wine using an old recipe book, but they do not like the taste, so Lupus gives it to Cornelius.
Toughy and Smarty have invented 'grasshopper tennis' (similar to volleyball), but whilst they play, Mouse floats out to sea on an old oil drum. Cornelius Button finally becomes aware of the boys being on the island, when he rescues Mouse.
Dr Hopper arrives threatening Cornelius to find another Button's Blue grasshopper, or he will take over the island. The boys ambush Dr Hopper, driving him away. The three boys then decide to stay permanently on the island.