When their regular babysitter is not available, a couple hires a friend of a friend to watch their three kids so they can have their anniversary dinner out. They should have checked references.
When the dad brings the new sitter to the house, he swaggers a bit too much. The mom is a bit too needy about her concern for the children. The eleven-year-old brother is a bully. They will suffer for their flaws.
The new sitter arrives with an eerie detachment. When the parents leave, and we find out what she's up to, the fates of the little brother and sister hang in the balance.
This is a psychological thriller, not a horror movie. Violence and gore are minimal. The sound editors do not resort to startling us with loud noises. The chills come from revealing the characters' intentions.
The movie has its flaws. Some minor characters are forgotten in the second half. There are some continuity errors in the run up to the climax. But the writing is better than average, and the performances, even from the little ones, are compelling.