The Drifter (1929)

Tom McCall, a deputy marshal, is detailed to hunt for narcotics smugglers and heads into the California Sierras where he buys an old white mule for $10 and goes to work for Ruth Martin, the owner of the Lazy M. The mule, which belonged to Ruth's recently murdered grandfather, is the only living being that knows the way to his hidden gold mine. Lawson, a narcotics smuggling aviator, attempts unsuccessfully to steal the mule and later, when the mule finds the mine, tries to forestall Ruth's filing a claim. When Tom hangs onto Lawson's airplane to make sure he gets to the claims office at the same time, the pilot does a few loop-de-loops to throw him off, but Tom manages to hang on. Back on terra firms, he arrests Lawson and files the claim for Ruth, whom he intends to make his bride.