Scientists surgically transform the Creature into an air-breather, but being able to live on land is not enough to make him comfortable with humans. Enraged, he turns his wrath on anyone who comes near as he desperately tries to return to the deep-water world where he truly belongs.
It's a bit up and down, but all in all 'The Creature Walks Among Us' is a film I'd consider as 'good'.
It, for me, definitively improves upon predecessor 'Revenge of the Creature', despite a few parts I didn't like as much as other bits. The stuff with the Gill-man is actually really good, especially towards the end. Despite that, the filmmakers seemed to prefer showing the humans - whose story didn't interest me on anything other than a base level. It needed more creature, in short.
Rex Reason is the only cast member I immediately remember, even minutes after watching. Leigh Snowden is serviceable, while Jeff Morrow at least plays his sorta character well enough - given I disliked him. Ricou Browning is the only main cast member to appear in all three times; as underwater Gill-man, of course.
The better sequel, though neither come all that close to the fab original.