Nutcase (1978)

Evil Eva and her cronies threaten to blow their enemies sky-high by reactivating the Rangitoto volcano with a nuclear device. However, the goofy gang isn't all bad - they like to break into song and dance while holding the city for ransom. Who will save the city from destruction? Why a talented trio of kids, including a nutcase who has invented an antigravity machine. The police force, zonked out on Eva's drugged coffee, are made ineffectual.