Let's Kill Uncle (1966)

Twelve-year-old orphan Billy discovers he’s inherited a vast fortune, but his excitement quickly turns to fear when he finds himself stranded on an island with his sinister uncle, a former British intelligence officer with murderous intentions. As his uncle employs an array of devious tactics—including hypnotism, shark-infested waters, fire, and toxic mushrooms—Billy’s only hope lies in the friendship of a brave young girl. Together, they must outsmart the cunning uncle and survive his deadly game before it's too late.