Invaders from Mars (1953), directed by William Cameron Menzies.
The scientist describes our young hero as a "cold scientific type not given to flights of fancy". I had no trouble being both at his age.
The paranoia science fiction thriller was big in the 1950s -- Invasion of the Body Snatchers (1956), I Married a Monster from Outer Space (1958) (a better film than its title) -- and even lasted into the 1960s with The Invaders (1967) TV series.
The formula:
Seeing strange things (which happened to me all the time, probably from watching these movies and reading cubic yards of science fiction. I was constantly on the watch for preternatural phenomena everywhere).
People changing (a recurrent nightmare where I was the last one left).
No one believes you (I was smart enough to not even try).
This entry is aimed at young people: David knows a top scientist and gets to use his giant observatory telescope. He mixes with the military men during the siege of the saucer site and penetrates the alien lair.
Critics at the time found this to be too scary for its audience and I admit it was creepifying and nightmare-inducing.
Some good aspects:
The saucer landing, the way it inserts itself underground without trace.
The sand pit trap is well done and always shocking.
The alien works have an eerie choir on the soundtrack.
The economical production values sometimes give a surreal, minimalist dreamlike quality.
It was probably done to pad the running time, but when David is running from the final explosion we have overlays recapitulating much of the film. This becomes projected against a background of stars which is... very strange.
Was the whole film a premonitory dream or a hint of something vastly more sinister?
Not so good:
Extensive padding using stock footage of tanks assembling.
The mutants are a handful of men in velour suits with zippers up the back. Maybe that's their uniform.
Some needlessly dumb astronomical errors. I'm pedantic on the subject but will spare my readers.
The odd:
Dad hits David hard, knocking him down.
The neighbor girl: they say she burned to death. That was suicide, which was bordering during the Code era.
At the observatory: they can see his house from there!
The alien boss as a head in a sphere: you would think that as cheezy but I'm willing to go with it.
We never know whether the implants were taken out of David's parents.
Shot mostly on a soundstage but we don't seem to mind.
The aliens have a specific mission, a reason for being here. In a sense they've joined in the Cold War and atomic competition extended into space.
Photographed by John F. Seitz -- Double Indemnity (1944), Five Graves to Cairo (1943), Night Has a Thousand Eyes (1948), Sullivan's Travels (1941), Sunset Blvd. (1950).
Available on Blu-ray.