Warning: This recap contains spoilers

The “Pretty Little Liars” theme song (“Secret” by The Pierces) has always creeped me out, but it’s particularly apt in the wake of this week’s #FatalFinale, the title of which is a nod to one of the song’s lyrics: “Taking this one to the grave.”

We’ve known that someone was going to be killed off and the episode opened with an ominous, but familiar scene: the Liars lined on a picturesque street, crying and comforting each other. It’s Thanksgiving and one Liar — Spencer — isn’t with the group. “Finally Spencer goes back to Radley…,” teased actress Troian Bellisario in a tweet this morning. “But in a way you’d never think. Had so much fun hate to say goodbye…”

That moment fueled the tension that hung over the episode, but in typical finale fashion, we didn’t learn who was killed off until the show’s final moments. But there were plenty of secrets revealed in the interim. Ali, the most notorious Liar of them all, took a polygraph test. There was a campy holiday scene in which we learned that Emily really likes Christmas and — as promised — Spencer returned to Radley.

At the end of last week’s episode, the girls came to the conclusion that Ali might very well be A. The Ali as A theory was confirmed by an unlikely ally: Mona.

(ABC FAMILY/Eric McCandless)

The girls paid Mona a visit at her house. “Why the grim faces,” Mona deadpanned. “Did somebody die?”

The Liars confessed that Ali was at the police station and they’re worried about what she’d tell them. It’s clear that Mona is their last resort for help. But she’s also a good choice. When she asks why they came to her, Aria pipes up: “Because you’re Mona.” And let’s face it, Spencer + Mona is essentially a brilliant duo. Mona later tells Spencer that she gained early admission to Harvard,  Yale, Dartmouth and Brown. And let’s not discount Hanna’s smarts (even though everyone else does). She apparently pulled a Zack Morris and did unexpectedly well on the SAT. She may not know her Churchill quotes or how grilled cheese works, but she can ace a college admissions test.

We got to meet Mona’s mother, who would be easy to dismiss as absent or oblivious given Mona’s detour to Crazytown a few seasons back. Au contraire, Mona’s mom seemed delighted that her daughter appeared to have friends, bringing tea and freshly baked cookies — Hanna’s fave, natch — out to the girls as they talked. As they parted ways, Mrs. V pulled Mona’s former BFF Hanna aside and asked if Ali’s return was causing trouble. Hanna pointedly told her that she was no longer friends with Ali, but acknowledged Mrs. V’s concern: “I do think that Mona’s afraid of her. We all are.”

After they left, Mona dialed an accomplice. “Send out a code A. She’s plotting something big.” It was Lucas, of course. Lucas informs Mona that Alison got to the army. “Crazy Mona, that’s what they’re calling you now.” Can I get a #PoorMona? Her reaction sets the tone for how the Liars will deal with their now-enemy, Alison. “I think she’s gone full on socio,” Mona tells Lucas.

Later, in the bathroom at Rosewood High, Mona breaks down Ali’s game. “She can’t play with her dolls if you won’t let her,” she tells the girls. They take turns wondering why Ali chose them for her game. “She assembled the perfect group,” Mona says. “Smart. Loyal. Admiring. Compassionate.” I feel like there should be an acronym here. Alas. Emily, not surprisingly, is the most hurt: “Bitch never loved me.” Mona instructs the group not to tell anyone that she’s helping them. “And that it includes your bed buddies.”

For Spencer, that’s Officer Toby, whose impending graduation from the police academy is apparently cause for role-play. Emily and Paige dance around the fact that Paige went on a date last episode, eventually sharing a kiss. Aria and Ezra also turn a new page — turns out Fitz is invited to the Montgomery family Thanksgiving. This is big.

That leaves Hanna to keep the operation a secret from Caleb, which is impossible. If Caleb can tell Hanna about seeing dead people, surely she can tell him that they’re working with Mona. Caleb joins Hanna, Paige and Emily for an intense afternoon of Christmas spirit, decorating Em’s house with all the trappings of the house you drive to see every holiday season because it’s so extravagantly lit up. Caleb senses that Hanna’s hiding something and she divulges every detail, which Mona is smart enough to anticipate. After teaming up with Lucas to hack into the Rosewood Police Department’s internal files, she calls a meeting to reveal what’s she got her hands on: a video of Ali’s polygraph.

Turns out Ali didn’t say anything that specifically implicates the Liars, but it’s clear that the police are focusing on Spencer and what they suspect to be her role in Bethany Young’s death. There are questions about Spencer’s amphetamine addiction and how loyal of a friend she was to Ali. Mona enlisted Caleb’s help in her little hackathon and he’s managed to find an affidavit that shows the police no longer consider Ali a suspect in the murder of Bethany Young.  They’re zeroing in on Spencer, who as the child of two lawyers, knows what the next step is: an arrest.

The group quickly hatches a plan to infiltrate Radley in an attempt to establish a connection between Ali and Bethany. Aria, continuing her “volunteer work” at the sanitarium, plays inside Liar, while Haleb watches from the outside with Hanna barking nonsensical codes into the ears of Mona and Spencer, both very familiar with Radley because they used to be patients there. Dressed as orderlies, they slip undetected (with Aria’s help) into the records room to find audio from Bethany’s therapy sessions.

Mona and Spencer at Radley. (ABC FAMILY/Ron Tom)

Meanwhile, Emily is tasked with occupying Ali’s time. She invites Ali over to her house under the guise that it’s Thanksgiving and they’re not talking and it’s really sad. As the Radley plan starts to go south, thanks to an appearance from Detective Holbrooke, Emily temporarily forgets that she’s supposed to be watching Ali. Alison asks for some tea and Emily turns her back long enough for Ali to discover frantic texts from Hanna.

“I’m A, right?,” Ali asks. Emily looks terrified and very, very angry. She says what we’re all thinking: “I don’t know. Are you?” Outside, Paige arrives with some chicken soup for Emily’s date night alibi: a scratchy throat. She sees Ali leaving in a huff and getting into the car with Mindy and Cindy, the twins formerly known as members of Mona’s army. Ali has gotten to them. Paige ignores the fact that Emily’s lied to her again and follows them in her vehicle.

Back at Radley, Spencer starts to put the puzzle pieces together — Ali’s mom was having an affair with Bethany’s father. With Holbrooke on their trail, they take the tapes and run out of Radley’s back door. Toby, the bed buddy with the least information about what’s going on, wants to know where Spencer is — she’s supposed to be at his graduation. Toby illustrates the perils of distracted driving, but he’s saved by the fact that there’s a full 10 minutes left in the episode. He emerges after the commercial break, alive but in a full leg cast.

As the Liars catch up over coffee the next morning (Thanksgiving) at The Brew, Paige drops in to tell Emily that Ali and the indy twins led her to an abandoned farm, where it appeared that Ali was forming her own army. Seconds later, Holbrooke and his partners arrive to arrest Spencer for the murder of Bethany Young. Spencer hasn’t quite figured out how to use the videotaped confession Melissa left for her, so for now, Bethany’s audio tapes are her best bet for clearing her name.

Enter Mona, who is listening to the tapes intently. She takes a break to send her mom off to Thanksgiving dinner, promising to follow after she finishes the “report” she’s working on. She senses she’s being followed and locks the door. She then calls Aria with a shocking discovery: Alison knew about her mom’s affair and lured Bethany to Rosewood. “She was jealous and wanted Bethany dead,” Mona says. “Alison is A and now I can prove it.” Aria and Ezra race over to Mona’s to see the proof, but someone else gets there first. A person in dark clothes with long blonde hair peeking out of the hood picks the lock on Mona’s door and seals her unfortunate fate. Arriving at the house, Hanna, Emily, Aria and Ezra make a grim discovery — there’s blood everywhere, but no Mona.

As Rosewood plays host to yet another crime scene, Holbrooke announces that despite the fact that there’s no body, they believe that Mona Vanderwaal has been murdered. The Liars, especially Hanna, are visibly upset. Ali appears in the crowd as well and it looks as if she’s smirking. It’s easy to think that the hooded blonde figure is Ali, but then again, CeCe Drake has proven that she can bears a passing resemblance to Ali. And anyone can sport a wig.

With no body, it looks as if we’re being set up to believe that Mona could make a return, a la her nemesis. But we see the undeniable proof — Mona’s lifeless body in a trunk. The murderer goes to Emily’s house and steals the baby Jesus from the Fields’s nativity scene, replacing it with a bespectacled Mona doll. That’s cold. The killer then places the Jesus on top of Mona’s body.

This redecorating session has apparently caused A to feel the holiday spirit — the hooded figure goes home to make a snow globe, complete with a picture of the Liars, snowflakes falling all around their happy faces.

Lingering questions: Who killed Mona? Does Jason know about his mom’s other affair? Is Spencer going to tell the police about Melissa’s confession? Can we stop pretending that Hanna’s dumb?