In Miss Bustier's classroom, Mylène and Adrien are holding water guns and hiding behind the teacher's desk. It is momentarily revealed that they are acting for a short film that Nino is making as an entry in a contest. Mylène is playing the role of Agent Smith while Adrien is playing the role of Officer Jones, and the two are devising a plan to take down a monster. However, the filming is interrupted when Mylène panics at the sight of Ivan's monster costume and screams.
To calm herself, Mylène sings "Smelly Wolf," but she backs up into Adrien and screams again. Chloé then makes fun of Mylène, making her run away. Adrien calls Chloé out on how mean she is, but Chloé dismisses it. Ivan runs after Mylène, trying to comfort her by saying that she's a better actress than she thinks and that the other students are being too quick to judge her. He gives her a pin from his favorite band, the Zombie Skull Crushers. Mylène puts the pin on her hat and thanks Ivan, but is still upset, and runs away to the bathroom, crying. Hawk Moth, sensing Mylène's distress, sends an akuma after her.
Back in the classroom, Nino rants to the class about how they treated Mylène and how because of that they no longer have their lead actress. Chloé then states that Mylène was a horrible actress and that she could do better; Ivan scolds Chloé for how she treated Mylène. As the producer, Marinette decides to go find Mylène, but the other students decide that until Mylène returns someone has to fill her role. Chloé wants to take the part because of a scene in the script where Agent Smith kisses Inspector Jones. Both Marinette and Alya freak out at the mention of this scene—Marinette because it could lead to Adrien and Chloé kissing, and Alya because she, as the scriptwriter, never wrote a kissing scene into the script. Nino reveals that he added the scene himself, and this infuriates Alya, who says that Nino had no right to change the script and that the new scene does not contribute to the overall story and the development of the central characters. Marinette stops the fighting and tells the class that she will find Mylène.
Right before Marinette enters the bathroom, the akuma arrives and embeds itself inside Mylène's Zombie Skull Crushers pin. Hawk Moth offers Mylène the chance to make others scared and afraid and she accepts, transforming into Horrificator, a creature that feeds on fear. After her transformation, she coughs pink slime onto the mirror and hides in a stall as Marinette enters the bathroom and calls out for Mylène. Walking into the room, Marinette notices the slime on the mirror and begins to investigate, but is interrupted by Alya, who enters the bathroom and warns Marinette that the scene where Chloé kisses Adrien is in the process of being filmed. Marinette and Alya return and interrupt the scene before Chloé and Adrien have the chance to kiss.
The students continue to argue. Chloé proposes turning Agent Smith into a nurse since she claims she would look good in a nurse's outfit. This irritates most of the students, but Marinette uses this as a diversion, suggesting that Chloé should go and find a nurse costume. Chloé leaves, taking Sabrina, Kim, and Max with her. Nino complains, as once again, they have no lead actress. Alya proposes that Marinette takes over the role of Agent Smith. Marinette protests this, but Alya quietly reminds her that this would be an opportunity to kiss Adrien. Marinette reluctantly agrees to take the role but says that Mylène will get to take over as soon as she gets back.
Meanwhile, Chloé, Sabrina, Kim, and Max are leaving the school as Horrificator watches from the roof. Chloé praises herself for the idea of getting a nurse's uniform. Arriving at the school's main door, Chloé realizes that Marinette is never a fan of her ideas, and suspects that they've all been tricked. She leads Sabrina back towards the classroom and tells Kim and Max to finish finding a nurse uniform. As Chloé and Sabrina leave, Horrificator sneaks up behind Kim and Max and startles them, causing them both to scream.
Back in the classroom, Nino starts filming Marinette and Adrien. Marinette says one line and leans in to kiss Adrien. Before they have a chance to kiss, Chloé slams open the door and interrupt, yelling at Marinette for tricking her, and saying that the movie needs her in it because her father is one of the judges. Suddenly, yelling comes from elsewhere in the school.
The students run out into the courtyard only to find pink slime and Kim's armband. Marinette realizes that the pink slime is the same as what she had seen in the bathroom, and suggests that they find Principal Damocles to talk about what to do. She notices Nino filming the situation and tells him to stop, but he refuses, suggesting that he can use this in his movie. Adrien heads back toward the classroom, claiming to be retrieving Officer Jones' jacket. As the other students run to Principal Damocles' office, Horrificator begins wrapping pink slime around the entire school.
In the classroom, Adrien removes his shoe—to make it look like he was taken by Horrificator—and transforms into Cat Noir. Marinette has a similar plan, dropping her phone in the hallway and transforming into Ladybug in the library.
Inside the office, the students only find pink slime and realize that Principal Damocles is missing. They head back out into the hallway and find Marinette's cellphone, presuming her to have been taken as well. They then realize that the entire school is covered in slime and that none of their cell phones has any signal. Ladybug arrives on the scene and tries to evacuate the students, but Cat Noir explains that slime is indestructible. He pulls Ladybug aside and reiterates the situation and what they have to do; Ladybug agrees completely. Nino films the entire scene.
Meanwhile, Horrificator carries Max to her lair in a maintenance area, holding him up by her tail. She drops him in a small pod of slime. As Max screams, Horrificator grows slightly larger, and Hawk Moth encourages Horrificator to absorb the fear of the students. Horrificator covers Max's pod with slime, trapping him inside.
As the students follow Ladybug and Cat Noir across the school, Chloé breaks away from the group with Sabrina, saying that it would be safer for them to hide and draw less attention. The students arrive back in the original classroom, where they find Adrien's shoe on the floor. Nathaniel is captured by Horrificator, who was hiding under the desk. Ladybug tells the other students to run, and all of them flee except for Juleka and Nino. Horrificator growls at Juleka, but Juleka finds Horrificator to be cool rather than scary. This causes Horrificator to shrink slightly, but she coughs slime at Juleka, who is saved at the last minute by Ladybug as Cat Noir distracts Horrificator from doing so. Nino starts to film their fight but is dragged out of the room by Ladybug. She and Cat Noir try to figure out where the akuma is hiding, but neither has any ideas.
Ladybug, Cat Noir, and Horrificator start to fight in the school's courtyard as the students hide near another classroom. Their screams cause Horrificator to grow, and Ladybug realizes that Horrificator is feeding on their fear. She explains this to Cat Noir and rushing at Horrificator, using her yo-yo as a shield. She restrains Horrificator with her yo-yo so Cat Noir can attack from above, but Horrificator pushes back Cat Noir with a spray of slime and pins him to the railing of the upstairs balcony. Pulling on the yo-yo string, Horrificator flings Ladybug into a basketball hoop and cements her there with slime as well. With both heroes restrained, Horrificator approaches the students but is distracted by Ivan. After staring at him, she licks his face affectionately as she takes Alix as her next victim and escapes into a nearby classroom.
Cat Noir breaks Ladybug out of the basketball hoop, expanding his staff to break the board behind her head. On the ground and free, she wraps her yo-yo around his baton and pulls him out of the slime. They try to pursue Horrificator, but are stopped by a door that has been secured shut with slime. Ladybug realizes that Horrificator is Mylène, based on how she treated Ivan. The group then realizes that Sabrina and Chloé are missing.
Meanwhile, Chloé and Sabrina are in Ms. Mendeleiev's classroom. Sabrina is barricading the doors with lockers while Chloé is painting her nails and praising her own ideas. Horrificator is revealed to be in the classroom, and she captures Sabrina and roars at Chloé. Chloé screams, alerting Ladybug and Cat Noir to where she is. Ladybug and Cat Noir use the slime left behind to track Horrificator to the maintenance area, bringing the remaining students with them.
Inside Horrificator's lair, pink slime drips in large globs from the ceiling and large pods rise from the floor. Ladybug calls out to find out who is there, hearing responses from everybody except Adrien. Cat Noir hides behind some of the pods to avoid being seen and calls out, saying that as Adrien that he is there. He then asks if Marinette is there, and Ladybug claims that everybody is fine. Both Ladybug and Cat Noir try to crack open the pods to no avail.
Horrificator enters and sprays slime on the exit, preventing anybody from leaving and dramatically landing right in front of Ladybug, who admits that Horrificator is getting quite scary. Cat Noir picks up slime pellets and throws them at Horrificator to distract her. Ladybug uses Lucky Charm, which gives her a set of guitar strings. Remembering that Horrificator is Mylène, she thinks of the song "Smelly Wolf." Off to her right, she spots a cart with industrial supplies on it and pulls it over with her yo-yo and hands out the supplies to the students while Cat Noir distracts Horrificator.
Blocking another bout of slime and climbing a ladder up to a second-story platform, Cat Noir uses Cataclysm on the steel poles holding the platform, trapping Horrificator in a makeshift cage. He jumps down from the platform and joins the students in their makeshift band. Rose is holding two trash can lids as cymbals. Alya and Nino hold traffic cones as microphones. Ivan has a brush and dustpan. Juleka is holding a box with a lid. Cat Noir uses old water containers and trash cans as a drum set. Ladybug uses an upside-down bucket, a broom, and guitar strings to form a bass. She then asks the others if they knew Mylène's lullaby, getting confirmation from everyone.
Ladybug begins the song, everyone else joining in on the second verse. The song causes Horrificator to shrink to a very small size. She runs to Ivan and jumps into his arms. Ladybug spots the pin on Horrificator's head takes it off, and crushes it between her thumb and index finger, releasing the akuma so she can capture and purify it. Throwing the guitar strings in the air, she cleanses the entire school and frees all the captured students. Nino keeps filming as Mylène returns to normal.
Hawk Moth, defeated again, claims he will destroy Ladybug as the window of his lair closes.
Nino films Mylène and Ivan kissing and uses this as the end of his movie. He shows it to Mr. Bourgeois, but the video is rejected from the contest due to the monster not being realistic enough. Adrien reassures Nino that this wouldn't mean the end of his career, as there are other directors who began from nothing. Alya offhandedly refers to the kiss not being the one they expected, and Marinette smiles, gazing off into the distance and the end card then appears.
Songs
Smelly Wolf
Characters
Major characters
Marinette Dupain-Cheng/Ladybug
Adrien Agreste/Cat Noir
Hawk Moth
Tikki
Plagg
Akuma
Mylène Haprèle/Horrificator
Ivan Bruel
Nino Lahiffe
Alya Césaire
Chloé Bourgeois
Minor characters
Sabrina Raincomprix
Lê Chiến Kim
Juleka Couffaine
Max Kanté
Rose Lavillant
Nathaniel Kurtzberg
Alix Kubdel
André Bourgeois
Mr. Damocles (background)