Much like Cthulhu from earlier this month, it takes the name of a monster from an H.P. Lovecraft story and just does a live-action remake of Shadow Over Innsmouth. This, however, is a much more faithful adaptation and is a lot better.
You know what's weird about Lovecraftian movies? Well, outside of the obvious? Two movies specifically reference two creatures in the title, but they're based on the story "Shadow Over Innsmouth." There's Dagon and today's movie Cthulhu. If I may reference Dr. Heinz Doofenschmirtz, "If I had a nickel for every time that happened, I'd have two nickels. It's not a lot, but it's weird that it's happened twice."