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Ok so TT starts out with kids in an industrial city setting, trick-or-treating. They knock on a door. No one answers, so they let themselves in, only to find an unexpectedly elaborate house, filled with dancing girls and fantasy characters. The PV ends with a knock on the same door they had just come through, along with Knock Knock's intro music.
Knock Knock starts with girls having a sleepover. They hear a knock on the door. They're disappointed to see that it's just an old man. Soon after shutting the door on him, another knock comes, and this time they're greeted by a book. They read the book together, which has pictures/stories of fantasy characters. Then it begins to snow so they run outside to play. Upon their leaving and staying out past midnight, the book comes to life, locks them outside, and then there' s a clockface that speeds backwards.The girls pound on the door to get back in, but vanish instead. Screen fades to the end of TT, with the kids inside the house w/the knocking at the door.
So given the backwards time thing this can follow, except... the girls weren't in the same house that the kids are? Though I suppose the house the kids end up in is "surprising" given the exterior, so maybe it's some sort of a portal situation.
When they were out in the snow it was reminiscent of a snow globe? This is the extent of my possible plot observations.