Welcome to the Canteen!
Two movies that did the same thing.
50s monster flick The Giant Gila Monster: for half the movie, livestock and people are disappearing, buildings destroyed, tension builds up as the characters (and audience) try to work out what the (unseen) cause is. (I won't spoil the surprise by revealing it here)
Don't Tell Mom the Babysitter's Dead: Having given away the plot in the title, they abandon it entirely and redo the plot of Working Girl, but actually do it better.
Thanks for the welcome (again)