The episode begins with Nooroo recounting the story of the Miraculouses, and explains that the Ladybug Miraculous and the Cat Miraculous are more powerful than the others and that whoever controls both Miraculouses will have absolute power. Nooroo is interrupted by a man holding a brooch with a picture of Emilie Agreste inside. The man proclaims that he wants that absolute power and asks Nooroo to remind him of the power of the Butterfly Miraculous. Nooroo explains the power of the brooch is to grant other people superpowers, and the man declares his intent to use it to create supervillains, thus drawing out the Ladybug and Cat Miraculouses with users to stop him. Nooroo protests, saying the Miraculouses are not intended to be used for evil. However, the man insists and forces Nooroo to submit to him. He attaches the Butterfly Miraculous to his chest, and he transforms into Hawk Moth.
Wayzz wakes up and flies out of a gramophone to Master Wang Fu, who promptly dismisses a man lying on a mat. As soon as the door closes, the kwami explains that he felt the negative energy of the Butterfly Miraculous. The brooch was originally thought to be lost forever. Determined to get Nooroo back, Wang Fu holds up his arm, with the Turtle Miraculous on his wrist. He calls Wayzz's name to transform but freezes in pain. When he stands up again, he comes to the conclusion that a new hero is needed. Walking to the gramophone, he presses several buttons, resulting in the gramophone revealing the Miracle Box of Miraculouses.
Inside her bedroom, Marinette wakes up. Turning off the alarm on her cellphone, she goes downstairs, kissing her mother on the cheek. Making breakfast, she bemoans her first day of school, expecting Chloé Bourgeois to be in her class for the fourth year in a row.
Her mother tries to reassure Marinette that she doesn't have bad luck, right before Marinette accidentally causes a chain reaction making a mess of all the food on the counter. Sabine silently reassures Marinette and helps her clean up. In the bakery below, Tom Dupain presents Marinette with a box of macarons to share with her class. Marinette hugs her father, kisses both her parents and runs out the bakery.
Marinette comes to a pedestrian crosswalk, waiting for the light to change. As she waits, Wang Fu crosses the street. Noticing a car heading towards Wang Marinette lunges forward and drags him onto the sidewalk. In the process, Marinette drops the macarons, and people step on them without noticing. She offers one to Wang, going on her way to school. Wang holds a box in his hand, picking up his cane and walking past the bakery.
Marinette runs past Fred Haprèle and into Miss Bustier's classroom. Miss Bustier is at the front of the class, telling Nino to move to the front row. He gets up as Marinette sits down, groaning. Chloé Bourgeois approaches Marinette and orders her to move, claiming the spot Marinette had sat in for the past few years is her spot, and that she wants to sit behind Adrien. Backed up by Sabrina, Chloé orders Marinette to sit in the front row next to Alya, a new student. Alya approaches, accosts Chloé and grabs Marinette's arm, dragging her away. In the process, Marinette trips over her own feet, dropping the box of macarons. When Marinette sits down next to Alya, Miss Bustier begins class. Marinette admits she fears she'll never be able to stand up to Chloé, and Alya replies with a quote from her favorite superhero, encouraging Marinette to stand up to evil. Marinette breaks apart the single remaining macaron giving half to Alya, as the two introduce themselves to each other.
Outside the building, Adrien runs toward the school, but a car swings around to meet him. Nathalie Sancoeur shouts at him to stop, as his father did not allow him to go to school. Adrien protests, but notices an old man is on the ground with his cane out of his reach. Adrien runs past Nathalie and his bodyguard to give the cane to the man and help him to his feet. When he turns around, Nathalie and the bodyguard block the stairs, ushering Adrien back into the car and driving him home. Wang Fu nods to himself, puts his cane on his shoulder, and walks away.
Back in Miss Bustier's classroom, Miss Bustier dismisses her class. As the students leave, Kim hands Ivan a note. Ivan reacts angrily, and Miss Bustier sends him to see Mr. Damocles.
Hawk Moth senses Ivan's anger and sadness and sends out his first akuma.
Outside the school, Mr. D'Argencourt leads Kim, Max, Alix, Nino, and Rose to class. Inside the school, Ivan opens the door to Mr. Damocles' office. Because he did not knock, Mr. Damocles makes Ivan leave the room and knock on the door. Angrily, Ivan leaves the room, and an akuma flies into the crumpled note. Almost immediately, Ivan accepts Hawk Moth's proposal of taking revenge on those who hurt him, and he transforms into Stoneheart. Mr. Damocles asks Ivan if he's knocking, and the door flies off its hinges. Stoneheart enters the room shouting Kim's name. The crash makes Mr. Damocles fall over, and students in the library also fall over with each step Stoneheart takes.
The students scream as they run toward the television in the library, Alya picks up Marinette and drags her by the hand. On the top left screen is Stoneheart, shouting Kim's name. In his office, Mr. Damocles calls the police and the army. Marinette has no clue what's happening, nor why the monster has Ivan's voice. Alya, on the other hand, claims that a superhero will show up to fight the villain, and leaves Marinette behind in the library, to chase after Stoneheart. Before Stoneheart leaves the school grounds, he picks up a car and chucks it at the school, knocking out the camera.
Wang Fu stands outside the Agreste mansion with a small box in his hands, smiling. Inside the mansion, Nathalie is asking Adrien questions about history, but Gabriel Agreste interrupts her, reprimanding Adrien for sneaking out and telling him that he has everything he could ever want in the mansion and that the outside world is too dangerous. Nathalie then states that they could be done with the homeschooling for the day if Adrien wished. Before she finishes speaking, Adrien runs from the room to his bedroom. Adrien lies on his bed, face in a pillow, but runs outside when he hears rumbling crashes. He sees police cars and hears sirens. The police ready their weapons as Stoneheart approaches, firing on Roger Raincomprix's command. When they fire, Stoneheart glows yellow, curling in on himself before growing in size. He picks up a police van and throws it as the officers scatter.
Marinette watches a news broadcast about Stoneheart's attack, terrified. Simultaneously, she and Adrien look down at the tables in front of them, each finding a small, octagonal, black and red box. When they open the boxes, each flashes with light. They both flinch backing away from the lights. From inside each box emerges a large sphere. Tikki appears in front of Marinette, who panics as Tikki tries to calm her down. Plagg appears in front of Adrien, who reacts with curiosity as Plagg flies around the room looking at shiny objects. Marinette calms down and questions Tikki, who begins to explain the situation. Adrien captures Plagg, who also begins to explain the situation. Marinette calls out to her parents, but Tikki tells her to stop, expressing friendship she asks Marinette to trust her. Tikki reveals to Marinette that she is the only one who can stop Stoneheart.
In Wang Fu's shop, while Wang Fu hides the jewelry box with its remaining Miraculouses, Wayzz asks Wang if he had made a mistake. Wang responds that he's only made one mistake in his life and he doesn't intend on making a second.
Marinette tells Tikki that she doesn't feel qualified to be a superhero and recommends Alya as an alternative, but Tikki insists that Marinette was chosen. Adrien tells Plagg that he can't be a superhero if he can't leave his house, but Plagg insists that can change. Marinette puts on the Ladybug Miraculous, and at Tikki's request says "Spots on!", panicking as she begins to transform. Adrien puts on the Cat Miraculous and says "Claws out!", beginning to transform as Plagg protests that he hasn't finished explaining.
Ladybug expresses her distaste at her new costume, and asks Tikki for advice, but can't find Tikki and is distracted by news footage of Alya following Stoneheart. She goes to the roof and throws her yo-yo forward, leaping into the air above the city.
Cat Noir walks along his staff between two buildings. He pauses as he hears screaming, and Ladybug falls from the sky and crashes into him, her yo-yo wrapping around them both, suspending them from his staff. They dangle as Ladybug apologizes. Cat Noir recognizes her as the partner Plagg had mentioned and introduces himself as Cat Noir. Ladybug tugs on her yo-yo to free it and it comes crashing down on Cat Noir's head. When she introduces herself, she calls herself "Madly Clumsy." Cat Noir forgives her for accidentally hurting him. Loud thumping interrupts them as a tall building crashes down. Cat Noir extends his staff and vaults up to Paris' rooftops, intent on saving the city. Ladybug reluctantly follows, screaming all the while.
At the Parc des Princes, the students are finishing up their class. Stoneheart jumps onto the wall of the stadium, shouting Kim's name. The students flee as Stoneheart chases after Kim, who trips over his feet and falls to the ground. Cat Noir's staff stops Stoneheart from touching Kim and gives Kim time to flee. Cat Noir goads Stoneheart into attacking him.
In his lair, Hawk Moth states that everything is going according to plan. Cat Noir hits Stoneheart with his staff, and Stoneheart grows in size. Cat Noir avoids Stoneheart's next blows as Ladybug watches in horror from the top of the stadium's wall. Stoneheart wrenches a soccer goal from the ground and throws it at Cat Noir. It bounces, arcing toward Alya, who is hiding and filming. Cat Noir acts fast, throwing his staff and extending it. The goal post bounces off it, missing Alya. This distraction gives Stoneheart the opportunity to catch Cat Noir. Alya spots Ladybug, asking her what she is waiting for. Ladybug winces and dives into the stadium. She hooks her yo-yo around Stoneheart's legs and slides to a stop in front of him. Unbalanced, Stoneheart releases Cat Noir. Ladybug runs to Cat Noir's side as he prepares to attack Stoneheart again. Ladybug stops him and reminds him of Stoneheart's ability to grow stronger with each hit.
Cat Noir suggests that they use their powers, and he summons his Cataclysm, testing it on one of the goal's posts, which rusts and dissolves. Excited, Cat Noir runs out to Stoneheart, placing his hand on Stoneheart's foot. When nothing happens, Stoneheart kicks him back to Ladybug, who berates him, telling him he has only five minutes left before he transforms back into his civilian self.
Ladybug summons her Lucky Charm, a wetsuit, and explains the akuma to Cat Noir, who points out that they don't know where the akuma is. Ladybug realizes that Stoneheart keeps his right fist closed, and suggests the akuma is hidden in his fist. She ties the open ends of the wetsuit together and ties a water hose into the neck of the suit. Asking Cat Noir to trust her, she wraps her yo-yo around his legs and throws him at Stoneheart, who catches Cat Noir in his left fist. Ladybug taunts Stoneheart and throws herself at him, wetsuit in hand. When Stoneheart catches her, he drops the crumpled note, which is now a purple rock. Ladybug then yells at Alya to turn on the hose, which allows the wetsuit to fill up with water and forces Stoneheart's hand open. Ladybug runs over to the purple rock and stomps on it, freeing the akuma, which flutters off as Ivan turns back to normal and drops Cat Noir to the ground. The crumpled note returns to its paper form.
Ivan has no clue what's going on or why he is at the Parc de Princes. Ladybug and Cat Noir pound fists, complimenting each other. Cat Noir leaves before his Miraculous runs out. Ladybug turns to Ivan, reading the note she finds it contains a taunting message from Kim to Ivan for not telling Mylène he loves her. Ladybug consoles Ivan as Alya films the entire scene. Alya asks Ladybug many questions as she's walking away and in response, she tells Alya to call her "Ladybug". Taking out her yo-yo, the hero now known as Ladybug swings up and out of the stadium.
In her room, Marinette watches a review of the fight on her computer. Her father calls her down from her room for dinner. The television in the living room shows a press conference from Mayor Bourgeois, stating that he will be hosting a grand party in Ladybug's and Cat Noir's honor. Adrien watches the same press conference in his room. While Plagg searches through the four covered dishes set in front of the television for food. He lifts the cover of the fourth one, revealing a dessert. Disgusted, he tells Adrien such and that his palate is more refined.
The free akuma lands on the top of the Eiffel Tower. It multiplies, sending out streams of its copies into Paris.
Marinette is washing dishes when the news reports on multiple citizens turning into Stoneheart. Tom and Sabine hold each other in horror as Marinette rushes to her room. At the same time, Adrien laments that he's going to smell like stinky old cheese because of Plagg's affection of Camembert. The news interrupts his lament explaining the situation with the people turning into Stonehearts. Except for they are inactive and immobile.
When they question their kwami on what went wrong, Plagg and Tikki ask the same question: "Did you capture the akuma?" Tikki explains that the akuma can multiply itself and seek out people who have negative emotions. This can create an army of akumatized villains. Marinette blames herself, saying it's all her fault and that she's too clumsy to be a superhero.
Tikki tries to tell Marinette that it's only her first time fighting, that she'll fight Stoneheart again and win next time. Marinette doesn't listen, insisting that she makes everything worse for everyone. She even states that Cat Noir would be better without her. Simultaneously, Marinette and Adrien learn that Cat Noir cannot purify akumas: only Ladybug can do it. But Marinette still tells Tikki to look for another Ladybug. She takes her earrings out, much to Tikki's protest. Before Tikki can say anything to dissuade Marinette, she disappears. Marinette puts the earrings back in their box and hides the box away in her vanity. She kneels at the foot of the vanity, upset.
Hawk Moth declares that Ivan's sensitive heart will allow him to be akumatized again. The akuma flies back through the open window, landing on Hawk Moth's cane and resting in its jewel. As the episode ends, Ivan sits on the steps of the collège, Adrien worries in his room, Marinette regrets her decisions in front of her vanity, and Hawk Moth cackles as his window closes.
Characters
Major characters
Gabriel Agreste/Hawk Moth
Wang Fu
Marinette Dupain-Cheng/Ladybug
Ivan Bruel/Stoneheart
Adrien Agreste/Cat Noir
Akuma
Tikki
Plagg
Minor characters
Emilie Agreste (pictured)
Nooroo
Wayzz
Sabine Cheng
Chloé Bourgeois
Tom Dupain
Fred Haprèle (background)
Caline Bustier
Nino Lahiffe
Rose Lavillant (background)
Lê Chiến Kim
Max Kanté
Mylène Haprèle
Sabrina Raincomprix
Juleka Couffaine (background)
Alya Césaire
Nathaniel Kurtzberg (background)
Alix Kubdel (background)
Nathalie Sancoeur
Adrien's bodyguard
Armand D'Argencourt
Mr. Damocles
Roger Raincomprix
André Bourgeois
Nadja Chamack
August's mother (background)
Civilians