<P> A knew that there was a very good chance Hanna would regain her memory . Hanna 's best friend , Mona Vanderwaal , informs the girls that she has also received texts from A , drawing the five closer together in trying to figure out who their tormentor is . Mona subtly encourages Spencer 's fears that her sister , Melissa , might be A as well as Alison 's killer . During Hanna 's recovery masquerade celebration , she suddenly regains her memory , revealing that Mona is A. However , she , Emily , and Aria ca n't do much about it , because Spencer and Mona are on their way to the police station . After Spencer is warned about Mona by text , she tries to escape Mona 's car , but Mona catches on rather quickly and diverts to a path in a more remote area of Rosewood . She tells Spencer everything , from seeing Ali launch a firework into Toby Cavanaugh 's garage and blind her friend Jenna , to finding Ali 's diary of secrets among a pile of old DiLaurentis junk the St. Germains left at the curb , discovering the girls ' secrets that only Ali knew . Her motivation was to get revenge for her friend 's blindness , even though Jenna and Ali had planned together to launch the firework . Mona did n't know this and got a scar on her stomach from the ordeal . Mona also drops the bombshell that Ian Thomas killed Ali , due to Ali 's last diary entry about giving him an ultimatum to break up with Melissa . She then offers Spencer to become A with her and tell Hanna that she must not be remembering correctly , but she refuses . The two fight at Floating Man 's Quarry and Spencer accidentally pushes Mona , who falls and has her neck caught between rocks . </P>
when is the first a revealed in pretty little liars
During Hanna 's recovery masquerade celebration