I think Edith Head deserves a credit in the Indiana Jones movies. The woman best known for her work with Alfred Hitchcock also designed Charlton Heston's costume here in the 1954 Jerry Hopper film Secret of the Incas. In addition to being an obvious influence on Raiders of the Lost Ark it's a very fun adventure film in its own right.
There are several adventure films in which one can see ancestors of Indiana Jones--the 1950 version of King Solomon's Mines, The Treasure of Sierra Madre. But Secret of the Incas not only has the look and the genre, it comes close to having the same plot, tone, and even chemistry between the male and female leads who are more contentious than was normal for films at the time.
Harry Steele (Heston) gets by running a tour guide racket in the Peruvian city Cusco. On the side, he hunts Incan artefacts and does various roguish jobs for a man called Ed Morgan (Thomas Mitchell).
I love the look of this bar where Steele meets with Morgan.
The movie takes place during a confluence of events that sees beautiful Romanian refugee Elena Antonescu (Nicole Maurey) enter Steele's life and a private plane enter his possession. In exchange for giving her a lift to the U.S. later, she helps him steal the plane so he can take it to an ancient temple where he hopes to plunder a legendary artefact before some legitimate archaeologists can.
There are a few musical numbers by the supposed natives that slow the movie down a bit, partly because one of them was used in The Big Lebowski and it is very difficult not to picture a topless woman on a trampoline followed by the introduction of Jackie Treehorn when hearing it.
But this sets up the conflict between Steele wanting to take the artefact for his own profit or acquiring it so he can give it to the people who worship it as an idol, a fundamental conflict present in the first two Indiana Jones movies.
Maurey is a bit of a cold fish as Elena, but Heston seems to have a lot of fun with his character, outwitting a hitman Morgan puts on him at the beginning of the film or casually handling priceless artefacts at the museum. His slowly awakening conscience at the temple is portrayed credibly and satisfyingly enough to make up for the somewhat limp portrayal of the natives.