Every day is a completely new day for Kristen (Nicole Kidman). A terrible car accident 20 years ago made her memory freeze on that day. The severe damage to her brain left her memory only for 24 hours. For Kristen, every day, she wakes up in a strange bed with a strange man lying beside her, and this strange man is her husband Ben (Colin Firth). Every day, Kristen calls Nash (Mark Strong), the doctor who treats her. With the help of Dr. Nash, Kristen gradually pieces together her own history. It turns out that at the end of each day, Kristen writes what happened that day into her diary, but gradually, Kristen finds that the content recorded in the diary is very different from what Ben told her. And one day, when Kristen opened the diary again, she found a sentence written in the diary: Don't believe Ben.