Season 2 (The Reveal)
In It's Alive A plants Ezra's stolen bachelor's diploma in Dr Sullivan's office for the liars to see during their session. At the end of the episode A is seen in Emily's home erasing her computer hard drive (which contains all the videos)
In My Name Is Trouble after Emily tears up the fake Danby letter and tosses it in her trash can, an unharmed replica of the letter gets sent to the Field's residence. Emily scrambles to her trash can to find the torn up letter, but instead finds a letter from A.
In Blind Dates A haunts Emily by sending her a box full of Danby paraphernalia (mugs and sweatshirts bearing the Danby logo) Pam is delighted as the A reference goes over her head. Later on in the episode after Spencer pawns Melissa's engagement ring for what she hopes is only 48 hours, A finds the cash to buy it from the Pawn Shop owner. After Hanna's visit to Dr Sullivan, A trashes the office and leaves a note on the wall saying NOSEY BITCHES DIE. Although the message isn't signed by A, it is pretty obvious based on Ezra's degree and the text message that A ravaged her office after Hanna's visit. At the end of the episode A stages the suicide of Ian Thomas. When the girls run to Melissa's aid, A puts Ian's phone in Spencer's bag and leaves unnoticed. This is the same phone A was using when texting Melissa pretending to be Ian.
In The Devil You Know it is revealed that A wrote Ian's suicide note. A composed the note from old A messages he/she sent to the girls. An also sends a text to Spencer telling her to admit the truth about Melissa's ring or there would be consequences. As Spencer is about to, A calls Ian's phone which Melissa finds and becomes convinced that Spencer was pretending to be Ian. In the same episode, A reveals to the girls via video that Ian may not have killed Alison. It reveals the full video of her and Ian on the night of her disappearance. It shows Ian getting up and walking away from a giggly happy Alison smiling at the camera. They go in search of the projector but A runs away, projector in hand.
"A" doesn't make any moves in Never Letting Go until the very end. She/he tampers with the tapes so that in the middle of the fashion show the screen pictures changes so as to make Alison look like the devil, with fire around and peeling black eyes. The screen flashes "the bitch is dead" and the music changes from sentimental to hellish. The girls flee from the run way as well as Jessica DiLaurentis. Spencer turns it off leaving the audience ill stricken. A later projects a note on the screen only when the liars are left which says "My Dresses, My Games, My Rules". As they leave it becomes clear that the candles at the show are in the formation 'A' Then in his/her room somewhere disclosed A purchases a pair of Tory Burch designer feminine, black boots online. Some fans believe that this means "A" is female but there is no proof that the boots were for himself/herself.
In Surface Tension "A" has a gift basket delivered to Hanna's house, stating the he/she knows that Emily and Hanna are living under the same roof ( Hanna declares the candy crappy) At the end of the episode A is seen using a syringe to siphon liquid Human Growth Hormone (HGH) and neatly release it inside Emily's tube of pain relief cream.
In Save the Date, "A" is seen to be under a sheet at the morgue where Spencer and Aria were earlier to get a look at Alison's autopsy results. He/She also sends Emily a message while she's in the hospital, letting her know that he/she introduced the hormones on her creams.
In Picture This, while Emily is with Samara and her friends, "A" sends her a message ordering her to get Zoey's phone number. Emily obviously had not wanted to, but since "A" had the results that showed the presence of the hormones in her body, she had no choice.
In Touched by an 'A'-ngel, "A" goes after Emily again, targeting her as the weakest link. She spikes her Alpha Bits cereal box to only contain "A's" and leaves her calling card in the plastic capsule meant for cereal box toys. Later, she crashes Emily's massage, giving her the massage instead of the professional without Emily realizing until she sees the letter A etched into the condensation of her water glass. At the end of the day, she takes Emily's psychological therapy session when Emily doesn't show up.
In I Must Confess, it is revealed that "A" had been a consistent patient of Dr. Sullivan. While Dr. Sullivan is perusing the files in her office, a black hooded figure is seen standing outside her window, watching the woman inside. Dr. Sullivan scans "A's" file, which shows that "A" has an obsession with revenge and refers to her enemies as "nosey bitches." The file also states that "A" wouldn't kill him/her self, only others. Dr. Sullivan calls the girls, wishing to tell them who "A" is. They show up to her office, but she is missing, something for which "A" takes credit in a text message. At the end of the episode, the black hooded figure is seen removing a sound device, which had been planted under the bobble head Freud in Dr. Sullivan's office.
In Over My Dead Body, "A" takes a seat opposite Dr. Sullivan in a booth at an unknown diner. Dr. Sullivan tells her captor that she's done everything you've asked. The black gloved figure doesn't reply, rather only hands her a manila envelope. Full of money? Dr. Sullivan looks disgusted and goes. Next, a waitress approaches the table where "A" is sitting, fills her coffee cup and inquires how her pretty-eyed patron is doing.
In the prequel episode, The First Secret, "A" sends Alison two texts, one when she is in the costume store and one at the end. From the second text, it becomes clear that "A" had posed in the doll-mask and burlap cowl while in The Haunted House and possibly also in the costume store. "A" had been the one to fight Alison with a butcher's knife and push her around. Alison had gotten away though when she kneed "A" and ran for it. At the end of the episode, "A" had taken off the mask, but his/her face was not shown; only a cellphone in hand was.
In Through Many Dangers, Toils, and Snares the four girls trick "A" by pretending to fight with each other over "evidence" of "A's" identity, which in reality doesn't exist. They hope this will reveal A's identity to them by forcing "A" to meet them in person. This plan fails, but they come away with "A's" cell phone.
At the beginning of A Hot Piece of A, someone calls the phone from a blocked number. When Emily is volunteering at a crisis center to fill her community service, she reads a transcript that sounds much like the situation that happened with Hanna, Aria, Spencer and Emily. When the person calls back again, Spencer and Emily's boss tells them to listen in and take notes. They realize that the caller is Lucas and he must be a helper for "A." At the end of the episode, "A" is seen fishing Lucas' shoe out of the river that he fell into after Hanna pushed him in.
In Let the Water Hold Me Down, Hanna and Mona get into an argument, resulting in Hanna running into the bathroom crying, she quiets down when she hears someone else enter, after a while she notices the sink is overflowing. She sees a small boat floating in it and a piece of paper that says, "Life is but a dream, Hanna. And I'm your worst nightmare - A". At the end of the episode, "A" is seen climbing some scaffolding and loosening a bolt in the wall of an unknown location, which is later revealed to be the barn in Spencer's house.
In The Blond Leading the Blind, "A" follows his\her orders by hurting Toby because Spencer made out with him. Toby has a broken arm and Spencer ends the relationship. At the end of the episode, "A" is seen cutting up pictures of Spencer/Toby, Aria/Ezra, and Hanna/Caleb, and burning them in a metal trash can. The final picture "A" burns up is not seen.
In A Kiss Before Lying, "A" threatens Hanna to stop Caleb from working on "A's" phone or "A" would expose her mom's secret of stealing money from the bank. At the end of the episode "A" is seen unlocking the drawer of Mr. Hastings' desk, and taking a gun out.
In The Naked Truth, "A" isn't seen until the very end where he/she walks by a sleeping Caleb and takes his laptop out of his bag.
In CTRL: A, "A" leaves a typewritten message on Byron's car, which has the info of Aria and Ezra's date on it. At the end of the episode, "A" is watching Emily through her window as she tries to call Maya.
In Breaking the Code, "A" sends messages to Mona, accusing her of shoplifting, and also a picture of Hanna's mom, Ashley, and Wilden the night the two got together. "A" also sends Mona Hanna's shoplifting report and threatens Mona to give them to the newspaper or she would be filling out a police report. Ashley and Ella, Hanna and Aria's mothers, find "A's" message and reports and become concerned about who "A" is and what "A" is doing. At the end of the episode, a gloved figure is seen shooting with the stolen gun at a target.
At the end of Father Knows Best, "A" is seen putting two quarters into the newspaper holder for the day's copy of the Rosewood Observer. She takes a paper featuring a huge picture of Maya on the cover with the headline "Missing Girl."
In Eye of the Beholder, it is assumed by the girls that "A" was the one who texted Jenna and brought her over to Jason's house, shortly after setting it on fire, with the intention for Jenna to die there. Hanna pulls Jenna out after coming back for Ali's bag Jason gave the Liars. The house blows up, but only the downstairs is damaged. The final scene shows a gloved figure scrummaging through the wreckage from the fire and planting a police badge at the scene.
At the end of If These Dolls Could Talk, Martha tells "A" she kept her end of the deal but the giant mess she has to clean up is much more than she bargained for. "A" hands her an envelope full of money, and gives Seth a lollipop. Seth is seen staring intently at "A."
In UnmAsked, the girls take a room in a motel. They stay in room 1--the same room Alison stayed in while she was alive—while, "A," lurking in the shadows, weaves in and out of their room without the girls ever knowing. She even visits while Hanna is in the shower, though "A's" intentions are unknown. She also spies on them from room 2, where "A" has presumably rented her own room.
At the costume ball, Mona shares with the Liars a story of how Alison, dressed as Vivian Darkbloom, had been spying on someone in a costume shop the week before she died. This prompts Spencer to realize that Alison had been searching for "A" all along and must have rented Room 1 in the motel in order to spy on the person in Room 2 - "A!" Together with Mona, Spencer sneaks the key from the desk and goes into Room 2 to find A's lair. Spencer is freaked out; the room is covered with pictures of all the Liars and Alison. There are Bratz Dolls that resemble each of the five girls perched in "A's" dollhouse.
Then, when Spencer spots a gum wrapper, the same gum that Mona offered her, in Alison's stolen diary, she suddenly becomes suspicious. When she sees the bag of cashmere sweaters, the understanding that Mona is "A" dawns on her. She turns around to confront Mona, who is now wearing her signature black hoodie. Mona knocks her out with a flashlight in return.
Next, Spencer finds herself riding in the front passenger's seat of Mona's car. Mona threatens her to join The "A-Team" or die at gunpoint from Mr. Hastings' stolen gun. Spencer gets Mona to reveal her motive. She is angry at the liars for stealing Hanna, the only friend she ever had. She claims to be over Alison and the way she had treated her. Spencer uses her phone to convey the message to the other liars, and Mona, without knowing it, is caught on video conversation for the other three liars to hear. Hanna is devastated when she learns that Mona is "A". Spencer lets the girls know where Mona is taking her by warning Mona that if she doesn't slow down, she won't make it to Look Out Point.
The girls move fast to go rescue their friend. Hanna catches up to the other two with her car and stops just short of hitting "A" with her car in anger. "A" runs towards Spencer and starts to fight her near the brink of a cliff. The other girls jet out of the car to help, and as a result, Mona falls several feet into a state of unconsciousness.
Later, the police declare "A" alive, and Dr. Sullivan returns to town to comfort the girls, revealing that "A" was threatening her son which forced her to flee. The police take her to a room with a one-way mirror. The girls look on with Dr. Sullivan who reveals that Mona has a personality disorder that causes her to be in a state of hyper-reality and mixed with her high intelligence level, it fueled her feelings of omnipresence and omniscience, while Mona has a monologue inside her head. She is unrepentant, cocky, and reveals that having the girls feeling safe and leaving their windows open and their doors unlocked is exactly what "we" wanted.
Later, Mona is sent to a mental institution. There, someone in a familiar red jacket comes to visit. Mona tells her: "I've done everything you asked me to," suggesting that "A," is in fact, working for someone else and Mona is perhaps part of a team of "A"s.