After Alison is confronted with a sudden, mysterious jeopardy, the Liars - infuriated by the continual harassment - decide to take things into their own hands for a counterattack. Hanna and Caleb work together to develop a plan to come face to face with the tormentor. Emily and Spencer team up to investigate if Mona had any involvement with the murder. Aria and Ezra continue to work on the book, though it might be more difficult than anticipated. Meanwhile, Ali encounters an unexpected visitor.