The four strangers who show up uninvited at a rustic getaway in the Pennsylvania woods, spouting biblical pronouncements about Armageddon and toting scary-looking homemade weapons (which they call “tools”) do knock before they barge in on Eric (Jonathan Groff), Andrew (Ben Aldridge) and their adopted daughter Wen (an adorable Kristen Cui).

And that’s after their leader, Leonard — played by Dave Bautista, in full gentle giant mode, wearing nerdy, wire-framed glass that appear too small for his boulderlike head — has introduced himself to Wen in the front yard, even going so far as to help her catch grasshoppers.

And when Leonard’s accomplices — excuse me, his “associates” — bust into the house and one of them (Nikki Amuka-Bird) whacks Eric in the head, severely concussing him, she immediately identifies herself as a nurse, offering first aid.

So have Amuka-Bird’s Sabrina, who came all the way from California; Adriane (Abby Quinn), a short-order cook from Washington, D.C.; and Redmond (Rupert Grint), a Massachusetts gas-company worker who is the only grouch in the bunch.

Based on the book “The Cabin at the End of the World” by Paul Tremblay (whose disturbing plot has been softened slightly by Shyamalan and co-writers Steve Desmond and Michael Sherman), “Knock” is satisfyingly atmospheric and tense.

Shyamalan keeps things deliberately vague and ambiguous — which actually contributes to the mood of delicious instability — but it’s also frustrating: If Leonard, a teacher, after all, is trying to make the case that he’s not nuts, he’s unsuccessful.