The Investigator (1973)

Episode 405 is Gerry Anderson's The Investigator 

The Investigator, a benevolent alien from a distant galaxy, selects an Earth boy and girl, John and Julie, to assist him in his mission to make their world a better place. The pair are miniaturized to assist The Investigator more easily, and assigned to prevent the theft by Stavros Karanti of a 14th century masterpiece from a church on Malta. John and Julie are presented with a car and a boat, scaled to accommodate their miniaturized size, and set out to thwart Karanti's plans..