The episode opens with Marinette and Adrien holding hands while at André Glacier’s ice cream stand, Andre talks about ice cream.
The scene blissfully changes to what appears to be the edge of the Seine, with Marinette and Adrien sitting next to each other sharing ice cream. After a long moment together, the two of them slowly look into each other’s eyes and try to kiss each other, but can’t due to the immense ringing of Adrien’s phone.
Panicking, Adrien quickly turns around and explains to Marinette that he’s supposed to meet his father, so he can’t stay with her – though Marinette doubts that’s true. After Adrien has left, Marinette turns to Tikki in confusion and asks her if Adrien is acting strange. Tikki shrugs and says that the two of them always act weird towards each other.
We soon see Marinette, Alya, and Nino sitting at a table together, with Marinette talking about Adrien’s strange behavior and how he randomly decided to leave without telling her beforehand. Jokingly, Alya and Nino mock her, asking how Marinette, of all people, the girl who should know Adrien’s schedule by heart, doesn’t know where Adrien is off tonight. However, Marinette defends herself, saying that Adrien can do whatever he wants and it’s not her fault. To answer Marinette’s previous concerns about Adrien, Alya pulls out her alliance ring and searches the Diamonds Dance, a social ball that Adrien’s father (Gabriel Agreste) hosts every year for his most powerful friends and their children. The group starts talking about how strict the ball is and that you have to be on the official guest list to get in, which means the likes of Kagami, Prince Ali, Chloe, and Zoe will be there.
The scene changes to Kagami and her mother fencing against each other in an open park until Kagami’s mother declares that they have to stop and prepare for the diamond dance. Unexpectedly, as she walked to the bench with her bag strung, a familiar voice called out her name. In a sad tone, Lila asks if Kagami is okay because he “doesn’t look happy”. Kagami then answers and admits that she is supposed to go to the diamond dance with her mother, but she doesn’t want to go because her mother and Gabriel Agreste planned to present her and Adrien as the “king and queen” of the prom.
Meanwhile, outside the movie theater, Zoé tells Marinette that she won’t be going to the Diamond’s Dance, saying that it’s just a place where hypocritical brats go. He then gives Marinette a white mask so she can go dance and not be recognized. At the place where the Diamonds Dance will take place, there are bodyguards scanning the white masks of the guests. Mr. Agreste rejects an incoming call from Lily Rossi on his Alliance ring. As he welcomes Kagami and Mrs. Tsurugi, a bourgeois family suddenly enters. When “Adrien” goes to the Diamonds Dance, Marinette appears and the bodyguards inspect her white mask.
Inside the place where the Diamonds Dance will take place, Amelie Graham de Vanily suddenly rushes in. When the bodyguards try to keep her out and fail, she pretends that Félix is still missing. Nathalie leads her out of the building and Marinette takes her mask to a machine that scans it and allows her to enter where the ball will be held. When she enters, she is wearing a white dress and a mask. As she pretends to be from an extremely wealthy family and invents a false identity when Chloé Bourgeois asks her questions, she sees “Adrien” and Kagami sitting together high above. As they talk about Adrien and Marinette’s relationship, they see their parents and Chloé’s parents sitting together in the corner.
As “Adrien” takes Kagami’s hand and the dance begins, Marinette runs up to “Adrien” and tells him that he could have told her he was going to the dance. Chloé and Mr. Agreste get furious that Marinette is at the party. While dancing, the robots scan Marinette and “Adrien” dancing, and Chloé realizes that it was the result of Zoé and Marinette working together. When Marinette takes a closer look at “Adrien” she realizes that he is not Adrien and is actually Félix and asks what he did with Adrien. Chloé jumps on Marinette and Félix transforms into Argos. He then releases Red Moon, a sentimonster, into the sky. Argos explains Red Moon’s powers and makes Chloé disappear as the dance guests flee in fear and terror. He then makes Mr. Agreste disappear, freeing him, his mother, and Adrien from him.
Argos snaps his fingers and makes them all disappear. He makes Mrs. Tsurugi disappear as Kagami is about to help her up. A devastated Argos carries Marinette and jumps out of the building where the Diamonds Dance is taking place. He takes Marinette to the basket and places her there. Marinette transforms into Ladybug. It appears when Argos snaps his fingers and makes random and innocent people disappear because the Red Moon is bigger and higher in the sky. Argos tells her his plans and wishes, which he makes when he and Cat Noir’s Miraculouses unite.
By this time the whole world will be bathed in the light of the red moon. Argos is about to snap his fingers and Ladybug uses her lucky charm and is shocked when she gets nothing. He looks at him and uses his Lucky Vision and realizes how to defeat him. Ladybug accepts her fate as Argos makes her disappear in a cloud of pink smoke. When Argos passes by, there is absolutely no one left. It’s no one, except Kagami, who rages at him. Argos brings Adrien back, leaving Kagami shocked. He tells Adrien and Kagami about how they are all in the free world now.
The scene flashes back to when Argos was on top of the building. He goes on a rampage that enters a ring that looks like Adrien’s ring and creates Red Moon. Marinette and Adrien are eating ice cream on Île aux Cygnes, just like at the beginning of the episode. Argos hides behind a tree and watches them. Adrien runs away and he hides with Red Moon in the locker room, calling out for Adrien. Adrien walks into the room and sees Argos. He tells Adrien to come closer and snaps his fingers, making Adrien disappear. Argos transforms into Félix and leaves, dressing and looking like Adrien, taking off his ring and putting on a ring that looks just like Adrien’s. He hides Red Moon much like Adrien hides Plagg.
The scene cuts to the present, where Adrien and Kagami are yelling at Argo, telling him to bring everyone back. He takes them both to where he hid Marinette. He is shocked not to find her there. He snaps his fingers repeatedly, trying to bring her back, but to no avail. Adrien and Kagami convince Argos to bring everyone back, so he does. Ladybug swings her yo-y to where Adrien, Kagami and Argos are and transforms into Marinette. He catches up with them and they rejoice and are reunited.