After her recent coronation, Twilight Sparkle and her friends set off for the Crystal Empire to attend a Royal Princess Summit. While her friends are buzzing with excitement, Twilight finds herself grappling with uncertainty about her title and the addition of wings to her form. Later that night, her world turns upside down when a cloaked pony steals her crown. In a thrilling chase, Twilight pursues the thief into the castle, only to discover that the culprit is none other than a fiery-haired yellow pony named Sunset Shimmer. During the chaos, the crown falls into a mysterious mirror, and the thief jumps in right after it.
The next morning, Princess Celestia informs the group about the identity of the thief; she explains that Sunset was her former pupil who became disillusioned and went through the mirror. This enchanted mirror, previously held in Canterlot, is now at the Crystal Empire and can only be accessed every 30 moons. With the portal set to close in three days, Twilight is entrusted with the mission to retrieve her crown. Though Twilight's friends are eager to assist, Celestia insists that this is a task Twilight must tackle alone. Ignoring her friends' pleas to stay behind, she bravely steps through the mirror, with Spike (in a moment of panic) trailing closely behind.
Upon arriving in this new realm, Spike morphs into a dog while Twilight transforms into a teenage human girl. Struggling to adapt to her new appearance, she soon finds herself standing in front of a large building, which she mistakes for a "castle". Entering, she encounters a group of teenagers and is helped up by a boy with blue hair. While exploring, she witnesses a bullying incident where she bravely tells a girl to stop her malicious behavior. That girl turns out to be Sunset Shimmer. Afterward, she meets Fluttershy, who explains that the crown Twilight is seeking had been turned in to Principal Celestia after Fluttershy found it.
Returning to the Principal’s office, Twilight learns that the crown is intended for the Fall Formal that takes place in three days. Discovering that Sunset has won the title for the past three years leaves Twilight feeling anxious about reclaiming her crown without causing an interdimensional incident. Determining to win the crown fairly, she decides to run for Princess of the Fall Formal.
In a busy lunchroom, Twilight confides in Fluttershy about her plan, only to discover that Sunset is quite hostile towards anyone competing against her. During this, Fluttershy reveals the necessity of winning over various cliques at school. While at the gym to meet the Party Planning Committee, Twilight unexpectedly runs into Pinkie Pie, and also encounters Applejack, who warns her about the challenges she will face.
Meanwhile, Sunset, along with her lackeys Snips and Snails, tries to undermine Twilight’s efforts by spreading rumors. Encountering difficulties with technology in the library, Twilight struggles with computers and copying machines but eventually finds solace with Spike, who makes her a bed out of books. As they explore Twilight’s yearbook, she notices the beautiful friendships among the girls—Fluttershy, Pinkie Pie, Rainbow Dash, Applejack, and Rarity—that seem to have faded over time.
The struggles continue when Twilight is humiliated by a video that Sunset Shimmer has posted online, showcasing her library mishaps. This leads to tensions among the girls, who start blaming each other for not being dependable. Through their discussions, they unveil the truth: Sunset may be to blame for their division.
Twilight decides to confront her fear and confess her royal background to her new friends. Surprisingly, Pinkie Pie manages to guess her secret, and rather than turning away, the girls rally behind her, willing to help. Together, they manage to fix the damage caused in the gym, and with the Fall Formal dance approaching, the atmosphere shifts towards hope and unity.
When the moment arrives to announce the Fall Formal princess, Twilight joyfully wins the title. However, the night is marred by Snips and Snails kidnapping Spike! Outside the gym, they discover that Sunset stands at the portal, threatening to destroy it unless Twilight surrenders her crown. In a defining moment, Twilight refuses, expressing that doing so would endanger not only her world but also the beings from this one.
As chaos ensues, with Sunset transforming into a demon-like entity wielding immense power, she attempts to seize control over Twilight and her friends. Yet in a stunning turn of events, Twilight channels the magic of friendship, igniting their inner power. The girls unite to defeat Sunset, freeing the students from her influence.
In the aftermath, Twilight chastises Sunset for allowing ambition to corrupt her, but extends a hand of friendship. With the school year concluding and Twilight preparing to leave this world, she shares heartfelt farewells with her new friends and steps back through the portal.
When she returns, her own friends are eager to hear her tales. Exhausted from the whirlwind of events, she decides to prioritize rest. As she walks down the hall, however, she unexpectedly bumps into Flash Sentry, a new member of the royal guard, igniting a new chapter of intrigue in her life.
