It was good, but wasn’t great. I’m a big B movie buff and this has a lot of elements of an A movie but still landed in the B category for these reasons (SPOILERS):
Ending: The pay off was lazy, and unrealistic. “The treatment center is right next to the hospital” was not good enough exposition to justify that ending. The plot was good though.
Evolution of Species: If you’re going to base the entity or animal on an “ancient species” at least explain the cuckoo theory thoroughly so that the audience leaves thinking this could actually happen. You could have used the music aspect way more impactful - like when they face off in the library, but the song is a “soft” one…
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Acting, Camera shots, art theory, sound, make-up, sci-fi, and general production were awesome.