It’s no secret that, when developing Raiders of the Lost Ark (1981), Spielberg and George Lucas pilfered liberally from the adventure films and serials they both loved as children. Even so, it’s quite remarkable how much the Indiana Jones series owes to Jerry Hopper’s Technicolor adventure romp, Secret of the Incas. Charlton Heston stars as Harry Steele, a roguish, fedora-clad treasure hunter, determined to discover a priceless Incan artefact hidden amid the ruins of an ancient Peruvian city.