When the Thompsons' regular babysitter can't come, they ask Anna, a friend of the babysitter, to watch their kids while they go out to celebrate their anniversary. At first, Anna seems like a dream come true for the kids because she lets them eat extra cookies and play with things that are usually off-limits. However, as her behavior gets stranger and stranger, the kids soon realize that Anna is up to no good and is not who she seems to be.