Quick recap
Gretchen and her stepbrother Clark stay with their grandparents in an old house in the middle of a swamp. The grandparents forbid them from entering one room upstairs. During hide-and-seek, Gretchen opens that door and finds a swamp monster. The grandparents drive off and lock the house. A note on the fridge explains: the monster invaded the house, the grandparents went for help, and if it gets out, the kids must kill it. Rotten stairs and a poisoned rhubarb pie don't do the job. The monster grabs Gretchen and licks her, then finds out it's allergic to humans and dies. Outside, the second note warns: stay out of the swamp, because the monster has dozens of siblings. Then the whistling starts.
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