Perfect murder

Harper and Ava are in a perfect relationship at the start. Ava tells Harper everything. Except one thing. She has not told her mom about Ava. On their one year Ava wants to come over to Harpers house for dinner. Harper is not ready to tell her mom and is scared Ava will tell her. But she does not want to disappoint Ava. Especially on their one year. Harper says yes and everything is cool. When Ava gets to the house everything takes a turn. When Harper opens the door everything goes blank. The last thing she remembers is Ava holding a knife. When Harper wakes up at the hospital she is so confused. She does not remember anything about herself. All she knows is Ava is the one who stabbed her. And Ava is still out there. How is she gonna stop her?