Maybe you need a refresher.
Review: "...a critical appraisal of a book, play, movie, exhibition, etc., published in a newspaper or magazine."
So, if you are reviewing a book called A Dog's Purpose, you don't reveal the dog's purpose. You let the author do that.
What you're writing isn't exactly a synopsis; it's a
Spoiler: "...a description of an important plot development in a television show, movie, etc., before it is shown to the public."