Candleshoe (1977)

For 10% of the action and a red Ferrari, she'd con her own grandmother.

When ex-con artist Harry claims that a secret treasure is hidden inside Candleshoe, an English estate, he creates an elaborate plan to find and steal the prize. By convincing a girl named Casey to impersonate the estate owner's long-lost granddaughter, Harry hopes to uncover the treasure's location. But when Casey has a change of heart, she must follow the clues and find the treasure, in order to save Candleshoe and stop Harry before it is too late.