Onion O'Brien is in trouble – again! This time he and the rest of the Five O's are trying to stop the destruction of the Valley, the local waste ground where they hang out. But Mayor Ronald Bump has other ideas – he wants to build new apartments, and he sets local guards The Ferg and Judge on the children.

As if things weren't bad enough, there's a new teacher who's acting very suspiciously. The Five O's can find out his secret if they manage to win the school talent contest. There's only one problem – they are completely talentless.

Then there's the discovery of the head of Ned Belly, and the local legend that claims he had hidden treasure …

Will the Five O's manage to outwit The Ferg and Judge, and Ronald Bump, and the new teacher, and solve the mystery of the head of Ned Belly, and save their precious Valley?