Mona Vanderwaal's faked death occured on 2011 Thanksgiving Day. She was revealed to be alive and held captive by A for five months (from late november to early may) this was revealed in the episode Welcome to the Dollhouse.
2 Days Before Thanksgiving
In Mona's bedroom, a French record plays, and a large collection of dolls sitting on shelves can be seen.
Leona tells Mona that she has company. Downstairs in the lounge, the liars await Mona, wanting her help. The girls are worried because Alison is taking a polygraph test and may not tell the police the truth. Spencer says that a lie is halfway around the world before the truth has a chance to get its pants on. Hanna states that the liars have been lying for Alison ever since she got back to Rosewood, so they need to know which story Alison is telling the cops. Mona asks why they think she can help them, to which Aria replies, "Because you're Mona".
As Hanna receives the cookies from Leona, Mona tells Hanna that she will see her at school, and goes back to her room. Leona asks Hanna if the bullying has started again now that Alison is back. Hanna tells her that she does not know what Alison is doing, as they are not friends anymore; however, she does believe that Mona is afraid of her, and adds that they all are. Back in her room, Mona dials someone and says, "Send out a code 'A'. She's plotting something big".
At The Brew, a worker opens the locked door for Mona. Mona tells the guy to lock the door, as she believes someone is following her. Mona is shocked that no one has turned up for her call, and asks where everybody is. Lucas arrives and tells Mona that they are not coming, as Alison has gotten to them and has gone back to calling her "crazy Mona". Mona looks on defeated before looking into the mirror and asking Lucas whether a person can be crazy and not know it. Lucas tells her that if she starts to believe this, Alison wins — however, Mona states that she is talking about Alison and that she has gone "full on socio".
Day Before Thanksgiving
In the bathroom at school, Hanna, Emily, Aria and Spencer have gathered to meet Mona. Mona tells them that Alison volunteered to take the polygraph test. Spencer tells Mona that Alison would never put herself at risk unless she believed her own story. Mona tells her that if you believe a lie, it becomes the truth. Hanna suggests that you lie to yourself if you did something so bad, you don't want to believe you did it; however, as Mona says, in Alison's case: she's playing a game. Mona thinks Alison took the test because she is not having fun anymore — she can't play with her dolls (the liars) anymore as they are not letting her. She also believes Alison is A.
Mona explains that Alison had assembled the perfect group of friends: smart, loyal, admiring and compassionate. Spencer says it was the perfect group for a sociopath to manipulate. Mona states that Alison would have gotten bored with any one of the liars, but all together, they are a challenge. Aria says that she never knew why Alison chose her as a friend and asks what made her so special. Mona tells Aria that when you don't feel anything, it must be really fun to mess with someone who feels a lot and that Alison picked Aria because Aria cares and Alison can't. Emily then asks if they are really saying that Alison is A out loud. They all exchange looks in agreement, Emily stating that Alison had never loved her. Aria adds that Alison had never really loved any of them.
Spencer tells the Liars that if Alison is ending the game because the Liars had cut her off, then Alison has gone to convince the police that she is innocent and that the Liars are the guilty ones. Aria asks what are they guilty of and Mona says "everything". Mona tells them that she can find out what Alison told the cops, to give them a head start on arranging their own stories. As Mona goes to leave the girls' bathroom, Hanna asks her why is she helping the Liars. Mona turns around and states that she got early acceptance letters from three colleges, but still needs to survive senior year, which she can't do alone. After they finish talking, Cindy and Mindy walk into the bathroom, and Mona greets them. The twins come back out of the stalls, exchanging mischievous smiles. Leona walks out of the principal's office and upon seeing Mona, lies to her about why she is in school. As her mom leaves and Mona walks to class, she sees Alison surrounded by a new group of friends. She smiles tauntingly at Mona before she is called to the principal's office.
At the police station, Lucas approaches Officer Maple and inquires about the high school liaison program. As Barry leaves to get a form, Lucas plugs in a Bluetooth to his computer and tells Mona, who is in the precinct's bathroom, to start hacking. Mona hacks into and acquires the files related to Alison's polygraph test.
Outside Emily's house, Emily, Paige, Hanna and Caleb are decorating the front lawn with Christmas decorations. Emily is bubbly about Christmas, as decorating is the one thing she does each year with her family, and she will not let Alison take that from her. Mona calls Hanna. As Emily puts a porcelain Jesus in a crib, Caleb asks why Emily loves Christmas and she states that it is always when her father comes home. Hanna lies to Caleb that the call she got was from her mom, but Caleb catches on quickly and says he knows that she's lying.
The liars and Mona watch the recording of Alison's test in Spencer's kitchen. Alison says she last saw Spencer come into her house. According to her answers, Alison was unofficially the leader of the group and the Liars presumably would have done anything for her if she needed them to. As far as she knew, Spencer was a loyal friend. Spencer was at her house and it was her fault as she pushed her to that place and that Spencer was angry. Alison found out Spencer was on drugs when she dropped her pills. Spencer was emotional, and Alison had promised her that she wouldn't say anything about it. She wouldn't know what Spencer would have done in return for her silence, as they never discussed the matter again. Finally, the instructor asks if she has anything else to say. The liars look on nervously as Alison states that she did not have anything to do with Bethany Young's murder.
Back at Radley, Aria is in the day room with some patients painting. She distracts a nurse by knocking over paint to allow Spencer and Mona to pass through the day room on their way to the file room. As Aria cleans up the paint, Holbrook approaches her, saying that she's made quite a mess of things. Holbrook tells Aria that he did not know she worked there, but Aria clarifies that she is volunteering. Holbrook questions when it all started, to which Aria asks what he means. He replies "the lies, the cover-ups, the murders". Aria looks confused, but Holbrook tells her "the volunteer program". Aria tells him that she started volunteering a few weeks ago; and Holbrook asks if this was before or after the reveal of Bethany's murder. Aria then objects, telling him that he does not sound curious anymore and wonders whether she needs to call her parents. Holbrook tells her not just yet before asking her one last question: whether the volunteer work was Spencer's idea. Aria answers with a firm no and Holbrook leaves.
Back at Radley, Mona and Spencer have found Bethany's records. They discuss colleges as they rifle through documents, with Spencer finding out that Mona had gotten early acceptances from four Ivy League schools before telling Mona that she hasn't heard from anyone yet. Mona offers her help before stuffing various papers into her bag, but Spencer says she think she's done enough. Spencer says to check Bethany's recording for August 8th. On the recording, Bethany claims that she thought "she really liked me but everything she really said to me was a lie, every word and every visit". Spencer clarifies that Bethany was talking about Mrs. DiLaurentis, who was having an affair with Bethany's father. Bethany questions if she can trust anyone in the DiLaurentis family and wonders if it is like mother, like daughter. Mona stops the recording and Spencer wonders if Alison had known Bethany.
Outside Radley, Hanna and Caleb notice Detective Holbrook going into the building. Hanna hastily alerts Spencer and Mona.
It's Thanksgiving Day at the Brew. They girls discuss Toby's accident, and that his broken leg will be in a cast for a month. Spencer says she told Toby everything about her situation. Paige arrives and tells the girls that she had followed the twins and Alison. They had driven out to an abandoned farm where at least a dozen cars were waiting. Paige tells the Liars that Alison is building an army of her own. At that moment Holbrook enters and arrests Spencer for Bethany's murder. The Liars object and try to stop him. They shout out that they will call Spencer's parents and that they will get her out.
At the Vanderwaal house, Mona is going through all the material they took from Radley while replaying Bethany's August 8th recording. Leona shouts for Mona to come down. As Mona leaves her room, Hanna texts her that Spencer has been arrested. Mona helps her mom load food into the car, as she and her mom will be joining a friend elsewhere for dinner, but Mona will head out after her mother. Mona watches as Leona drives away and head back towards her house, but hears ruffling in the bushes and stops to investigate. She goes into her house and locks the door while a figure in black with blond hair watches.
At the Montgomery house, Ezra arrives with the pies and Aria tells him that Veronica has not seen Spencer yet as she is still being processed. Ezra assures Aria that they are working on her bail and they will have Spencer out of jail in no time. At that moment, Mona calls Aria and tells her that Alison lured Bethany to Rosewood, as she knew about the affair. Alison set her up because she was jealous and wanted Bethany dead. As Mona is in her room talking on the phone, Mona's Kidnapper picks the lock on the front door and enters the house, creeping up to her room. Mona tells Aria that Alison is A and she can prove it. Aria hangs up the phone and tells Ezra they are going to Mona's.
In Mona's bedroom, Mona looks into the mirror and says "Game over, Alison. I win." Mona's Kidnapper enters the bedroom and Mona, thinking it's her mom, turns around and gasps, falling back onto her dresser. Hanna, Emily, Aria and Ezra enter the house to see blood everywhere and the place trashed. Hanna shouts for Mona. Hanna and Emily run upstairs to see if Mona is there while Aria surveys the living room full of blood. The scene cuts to outside the house as a long scream is heard.
The police eventually arrive and have taped the house off. Leona looks concerned while the liars huddle, crying and shocked. Hidden between the news vans, Alison looks on smugly. Holbrook announces to the reporters that the investigation is ongoing and the incident has been ruled as a homicide. Although the police did not find a body, the amount of blood indicated the victim's wounds were fatal. Holbrook then declares that Mona Vanderwaal was murdered and Leona breaks down, crying. The liars are even more upset and Lucas has arrived, also shocked. Back at the vans, Alison surveys everyone's emotions before turning around and walking away.
Outside the Fields' residence, the Christmas decorations are all lit up. The figure approaches the nativity set and takes Jesus out of the crib and replaces him with a Mona doll. The figure takes the Jesus doll to their car and opens the trunk, revealing an unconscious Mona. The figure then places the Jesus doll on Mona's body. We get a close up of Mona's face and it is revealed that she was crying before she fell unconscious. Since it is later revealed she is alive and was being held hostage by 'A', it would be appropriate to assume she had been drugged by 'A' and that is the reason she is not moving in this shot.
"Taking This One to the Grave" closes with A cutting a picture of the four Liars out and putting it into a snow globe. A seals the bottom and shakes the snow globe before putting it down near the Christmas tree.
- Alison is arrested for Mona's murder in "Through a Glass, Darkly" and subsequently put on trial in "To Plea or Not to Plea". She was found guilty in "I'm a Good Girl, I Am".
- Hanna is arrested as an accomplice to Mona's murder in "To Plea or Not to Plea". Aria, Spencer and Emily were also arrested as Alison's accomplices in "I'm a Good Girl, I Am".
- Mona was revealed to be alive in "Welcome to the Dollhouse" and was being held captive by "A" in the dollhouse for five months.
- Tanner finds out that Mona is alive and has been held captive in the dollhouse, meaning that none of the Liars are guilty of anything.