In this special, the Sunset Shimmer of the human world enacts revenge on her pony counterpart and does everything she can to ruin her life, and it's up to Princess Twilight and her friends to help stop her.
The special begins with the students of Canterlot High School, including the Mane Seven, getting ready for another Fall Formal. It's the last Fall Formal before the girls graduate, and they're looking to make it the best one ever, especially since it will be Twilight Sparkle (Sci-Twi)|Twilight Sparkle's first and only Fall Formal. Sunset Shimmer thinks back to the last Fall Formal where she turned into a "raging she-demon," but her friends remind her again that it's all in the past. Sunset reveals that she's not running for Princess of the Formal this time around, so Twilight tells the girls that she's going to try running. The girls excitedly help her prepare to do so.
Later, Sunset writes to Princess Twilight Sparkle via her journal and recaps on what's been happening. She tells her that she hopes she never has to leave her friends. She then posts a picture of her and her friends onto MyStable with a similar caption.
- "I don't know what I would ever do without my friends. I hope I never have to leave them."
- —Sunset Shimmer's MyStable post
After school lets out, Fluttershy is on her way to the animal shelter, when suddenly, she is thrown into an alley by someone. The figure reveals herself to look and sound exactly like Sunset Shimmer. Unbeknownst to Fluttershy, this Sunset is native to the human world and is only pretending to be the Equestrian Sunset. The human Sunset throws Fluttershy around and tells her to warn her friends that she's coming for them as well. She then leaves her in the alley in tears.
The next day, Sunset runs into Fluttershy in the halls of CHS, expressing shock at her injuries. She tries to ask what happened but Fluttershy runs away in fear. A confused Sunset asks her friends what's going on with Fluttershy, but they immediately act hostile toward her, as Fluttershy had already told them about the incident. When Sunset's friends accuse her of attacking Fluttershy, she denies it, claiming she doesn't know what they're talking about. Sunset tries to convince her friends that it wasn't her, but they walk away, leaving her in bewilderment.
Later, Sunset finds Fluttershy and reassures her that she won't hurt her. Sunset uses her geode to see what happened, and later she writes to Princess Twilight about the situation.
Return of the Princess
In the days that follow the attack on Fluttershy, Rarity and Applejack find the human Sunset destroying both the Apple family's barn and the Fashion Boutique. They try to stop her, but they are blown off their feet by a yellow magic wave. The human Sunset also tells Rarity and Applejack to warn the rest of the girls of her coming.
The two girls confront Sunset again the next day and berate her for the incident. Again, Sunset tries to tell them that she's innocent, even bringing up already going through "the incident with the MyStable account." Applejack and Rarity don't listen to her and warn her that there will be consequences if another incident happens again.
As Applejack and Rarity walk away from Sunset, she quickly writes to Princess Twilight that she needs help. Later, Sunset meets her as she comes through the portal outside Canterlot High. Sunset recaps Twilight on what she knows, and Twilight tells her that it may be another Sunset Shimmer, one native to the human world. However, Sunset dismisses the idea, claiming it to be impossible.
Twilight then goes to the Mane Six, who immediately warn her about Sunset. She starts chewing them out about jumping to conclusions and not hearing Sunset out, but they shut her down and tell her that they don't trust her. Little do they know that the human Sunset had been eavesdropping on their conversation. A miffed Twilight leaves to rejoin Sunset, and tells her that she will investigate further.
- "Don't worry, Sunset. I promise you, I will figure out what going on here."
- —Princess Twilight reassuring Sunset Shimmer
That night, Sunset finds a note with her cutie mark on it on her bed, which reads, "There can only be one."
Princess Twilight accompanies Sunset the next day, to make sure she knows where she is. They go through the day without anything happening, but it's revealed after the day ends that the Mane Six were helping Twilight with her campaign for Princess of the Fall Formal without Sunset. Sunset angrily tells them again that she's innocent, but the girls don't want to hear it. Princess Twilight tries to get them to listen to Sunset's side of the story, but the girls tell her they don't need it. Twilight storms off with Sunset in tow.
Later, Sunset and Princess Twilight come across a smear campaign made to humiliate Twilight Sparkle, and they realize it is very similar to the one Sunset did for Princess Twilight when she ran for Princess of the Formal. Hearing the Mane Six coming, they immediately start running, with the girls chasing after them. The girls corner Sunset, and Princess Twilight puts herself in between her and the enraged girls. When the girls try to push past Twilight to get to Sunset, she snaps at them, stopping them in their tracks. She tells them that they're going too far and that they're not acting like the bearers of the Elements of Harmony. The girls finally walk away, and Sunset tells Twilight that the situation is getting out of hand. Twilight tries to reassure her that they'll figure it out, but Sunset is less than certain.
Twilight lets Sunset go back to her apartment, and she walks around town thinking about the situation. Just then, she runs into the human Sunset, who immediately pretends to be the Equestrian Sunset. Twilight sees through the act, and the human Sunset runs away. Twilight tries to catch her, but is blown back by another yellow shockwave. As the human Sunset escapes, Twilight wonders what caused the blast.
Twilight catches up with the Equestrian Sunset and tells her all about her encounter with the human Sunset. The two go to the Mane Six and tell them about what they saw, but once again, they don't believe them, also deeming that another Sunset would be impossible. Sunset loses hope and walks away, with Twilight following her.
In the days that follow, Sunset is continuously met with rage and accusations of more attacks, including humiliating Rainbow Dash at a soccer meet and destroying Pinkie Pie's party supplies. When the human Sunset crashes one of Pinkie's slumber parties the night before the Fall Formal, The Mane Six finally snap and start screaming at Sunset the next day, Princess Twilight having no way of stopping them. Sunset finally breaks down ino tears and does the unthinkable: She admits to the incidents, much to Twilight's shock. The Mane Six tell her to never come near them again, and leave her and Twilight in the hall. Sunset tells Twilight that she only confessed because she couldn't take any more of the accusations. Twilight runs off to confront the Mane Six once again, but stops when she hears Sunset called to Principal Celestia's office.
When Twilight arrives, Sunset has been expelled from Canterlot High. Twilight begs Principal Celestia to reconsider, but her decision has been made. When Principal Celestia mentions that Sunset had been reported by a student, Twilight asks who it was. Principal Celestia hesitates before answering, "Twilight Sparkle."
Princess Twilight finds her human counterpart alone in the halls, and asks her if she reported Sunset. Twilight tells her that she did what she had to, and that Sunset is no longer her friend. Princess Twilight is taken aback by her cold responses, and tells her that Sunset only confessed because she and the girls blackmailed her into confessing. When the rest of the Mane Six appear, Princess Twilight repeats what she said, but the girls are just as cold as Twilight. Twilight tells her Equestrian counterpart that she put too much trust in Sunset and that they were all wrong about her. Princess Twilight snaps back at her, saying that she was wrong about them. Twilight simply responds with "It's over."
Princess Twilight storms away from the girls, but when she gets out of the school building she realizes what she's done. She collapses to the ground in tears. Sunset walks out of CHS in a daze to find Twilight sobbing on the ground. She helps her to her feet and takes her back to her apartment. The human Sunset watches in the shadows, grinning that her plan is nearly complete.
That evening, Sunset and Twilight sit and watch Canterlot High from across the street. Sunset tells Twilight that there's no point for her to stay in the human world anymore, but she's suddenly interrupted by the human Sunset, clad in a black armored suit. She laughs and tells Sunset that her life is almost completely ruined, but there's still one more thing she has to do. Twilight realizes that the human Sunset is going to attack the Fall Formal that night. The human Sunset suddenly runs away, and Sunset and Twilight chase after her. The human Sunset runs back to her house and leaves the door open for Sunset and Twilight to run in. Using the source of the magic blasts from earlier - a magic yellow jewel - she knocks them both unconscious.
That night, the Fall Formal gets underway at CHS, with Flash Sentry's band doing another live performance. At the human Sunset's apartment, the human Sunset puts some finishing touches on her suit, and puts the jewel in the center of it. She then goes to Sunset and Twilight, who are gagged and tied to chairs. The human Sunset shows off how the suit allows her to use her gem's powers, before telling Sunset that "turnabout is fair play." She then leaves the two girls bound in her apartment before leaving for CHS.
The human Sunset gets to CHS right before the Mane Six does, and she overhears their conversation on if they were right about abandoning Sunset and Twilight. She then climbs up to the roof, waiting for the right time to strike.
- "This is gonna be fun."
- —Human Sunset after listening to the Mane 6's conversation
Back at the human Sunset's apartment, Sunset and Twilight manage to get their gags off and, using Sunset's journal, whch was on a desk inside Sunset's backpack, they write an SOS back to Equestria. Starlight Glimmer, Spike, and the pony Mane 5 get the message, and leap through the portal to go save Sunset and Twilight. The pony Mane 5 are disoriented and freak out at first, but Starlight and Spike help them get grips with their new bodies. Using Sunset's directions from her message, they free Sunset and Twilight and run back to the school.
At CHS, Principal Celestia announces that Twilight Sparkle is Princess of the Fall Formal, but suddenly, the human Sunset crashes through the ceiling and confronts the Mane 6 once again. She grabs Twilight in a magic field and begins to choke her, telling the surrounding students that they were fools to trust her. Just then, Sunset, Princess Twilight, Starlight and the pony Mane 5 show up. The two Sunsets stare down each other, but when Sunset tells her counterpart that her plans are ruined, the human Sunset flies into a rage and rapidly fires yellow blasts of magic at Sunset. Despite the human Mane Six's attempts to stop her, the human Sunset catches Sunset and tells her:
- "If I can't ruin your life, I'll just end it."
- —Human Sunset to her counterpart
Sunset then rips the human Sunset's jewel out of her suit and throws it across the gym. The two Sunsets engage in a fistfight, both of them evenly matched. The human Mane Six then use their geodes to knock the human Sunset into submission, also throwing her across the gym. Sunset asks her if she gives up, but the human Sunset, who had fallen onto her gem and put it back into her suit, tells her that she never gives up.
The human Sunset then stabs Sunset through the chest with a magic wrist blade, as everyone in the gym watches in horror. In Equestria, Princess Celestia senses the event and collapses in shock, Princess Luna worriedly asking what's wrong. The human Sunset looks into her counterpart's eyes and says, "It's over."
The human Sunset dissipates her blade and Sunset collapses to the floor. Princess Twilight immediately runs to her and begs her not to go. In her final breath, Sunset wheezes, "Never forget..."
- "Sunset! Nooooooooooooo!!!!"
- —Princess Twilight mourning Sunset Shimmer
As Twilight and the Mane Six mourn the loss of Sunset, the human Sunset sneers at Twilight, telling her that neither she nor Sunset should have come to the human world. Twilight angrily asks who she thinks she is, to which the human Sunset responds, "I'm Sunset Shimmer. And don't you forget it."
The human Sunset then leaves the school without another word, leaving the students of Canterlot High to their sorrow. In Equestria, Princess Celestia sobs while Princess Luna comforts her.
Elsewhere, the human Sunset sits on a dock overlooking a lake. She looks at her phone, which displays Sunset's MyStable post from a few days before. She then proceeds to take her gem out of her suit and hurl it out into the water. She then sits in the moonlight with a conflicted look on her face.
- Sunset Shimmer: I'm gonna let other people run this time around. I've taken Princess of the Formal for these past few times.
- Pinkie Pie: Well, except for when—
- Sunset Shimmer: Yes, I know, except when I tuned into a raging she-demon.
- Human Sunset: Run back to your little friends, Fluttershy. Make sure they know that I'm coming for them.
- Applejack: What th' hay are you doin' to my barn?!
- Human Sunset: Don't you know destruction when you see it?
- Sunset Shimmer: Why do we have to go through this again? Wasn't the incident with the MyStable account enough?!
- Princess Twilight Sparkle: I've been analyzing what you know and what seems to be going on, and the only conclusion I can make... is that it's another Sunset.
- Sunset Shimmer: What?
- Princes Twilight Sparkle: Another Sunset Shimmer. One native to this world. Your counterpart.
- Sunset Shimmer: That can't be. That's impossible.
- Princess Twilight Sparkle: This is going way too far, girls! This is not behavior you would see in the bearers of the Elements of Harmony!
- Princess Twilight Sparkle: You're not her. You're not the Sunset I know.
- Human Sunset: There's only one Sunset Shimmer.
- Princess Twilight Sparkle: That's what you want me to think!
- Principal Celestia: If I wasn't notified that she was causing trouble, someone could get seriously harmed.
- Princess Twilight Sparkle: She's not causing— wait, notified? Who notified you?
- Principal Celestia: [sighs] Twilight Sparkle.
- Twilight Sparkle: It's over.
- Human Sunset: You ruined my life, and now I'm returning the favor. Turnabout is fair play, after all.
- Pony Rarity: What the— what am I?!
- Pony Applejack: Well, this is different.
- Pony Rainbow Dash: Where are my hooves? Where are my wings?!
- Pony Fluttershy: Someone help me...
- Pony Pinkie Pie: Girls! Calm down! We're just humans!
- Princess Twilight Sparkle: Girls, meet Sunset Shimmer.
- Pony Pinkie Pie: Hi! I'm Pinkie Pie! We're heard a lot about you, including how how stole Twilight's crown and used it to become a power-crazed she-demon, defeated a bunch of sirens with the power of music, brought back your Twilight Sparkle from being consumed by the darkness inside her, helped saved a camp using those super-awesome geodes, got trapped in a mirror and was saved by Starlight, brought back your friends' memories of you after someone erased them all, used the Storm King's staff to save a vacation cruise from a deadly magic storm, and got trapped in a time loop during a music festival! Did I forget anything?
- Sunset Shimmer: No, that's pretty much it.
- Principal Celestia: I give you your Fall Formal Princess... Twilight Sparkle.
- Twilight Sparkle: Thank you all so much.
- Human Sunset: Oh yes, she's so very special.
- Human Sunset: You just had to show up, didn't you.
- Sunset Shimmer: What, you thought I was just gonna sit there and let you ruin my life?
- Sunset Shimmer: Your plans have been ruined, Sunset. Now what are you gonna do? Well?
- Human Sunset: What I should have just done from the start.
- Sunset Shimmer: Do you give up or do we have to keep doing this?
- Human Sunset: There's only one thing you need to know about me... I never give up.
- Human Sunset: It's over.
- Princess Twilight Sparkle: Please don't go...
- Sunset Shimmer: Never... forget...
- Princess Twilight Sparkle: Just who do you think you are?!
- Human Sunset: I'm Sunset Shimmer. And don't you forget it.