It's Marinette's first day of school. Hawk Moth captures a butterfly kwami named Nooroo and forces his allegiance. Hawk Moth's butterfly brooch allows him to give someone a superpower they desire and to make that person obey his will. He plans on using his new powers for evil purposes, intending to create villains to lure the heroes out to fight them, so he can steal their Miraculouses and gain absolute power. Master Fu, a 186-year-old Chinese man,is notified of Hawk Moth's capture of Nooroo through Wayzz, his own kwami, a turtle. However, he is too old to fight anymore, so he pretends to be a weak old man, travelling Paris to find suitable charges for the Black Cat and Ladybug Miraculouses, hoping they can stop Hawk Moth and free Nooroo. He is helped by Marinette when he crosses a road and is almost hit by a car, and Adrien when he pretends to have fallen down. As school is dismissed, Ivan and Kim, two of Marinette's classmates, have a dispute, after Kim sends Ivan an offensive message on paper. Upset and angry, Ivan becomes Hawk Moth's first ever victim, turning into Stone Heart, a giant rock monster who grows in size after taking damage. Master Fu learns of this, and has no choice but to quickly deploy both the Black Cat and Ladybug Miraculouses, sending them to Adrien and Marinette. Marinette and Adrien both find a small box in their room, and after touching them, are quickly greeted by Tikki and Plagg. Both kwamis give their charges a quick explanation; soon Marinette and Adrien transform into Ladybug and Cat Noir and rush to stop Stone Heart. Despite their lack of experience and poor fighting skills, they manage to defeat Stone Heart and free the akuma. However, Ladybug forgets to purify the akuma after the fight; it flies off, duplicating itself and infecting many people, turning them into immobilised Stone Hearts. The episode ends as Marinette learns of her fatal mistake, and in her despair, thinking that she can't do her job as Ladybug, removes her earrings and puts them away under her dressing table, causing Tikki to disappear.

Nino is a contestant in a game show called The Challenge, where he has to get Mayor Bourgeois to dance. Despite being invited to see Nino, Marinette is grounded by her parents for missing too many classes at school, not knowing that she had been working as Ladybug. As Nino completes his challenge, the next contestant, a hypnotist named Jacques Grimeau, is given the challenge to hypnotize Adrien's father Gabriel Agreste. However, Gabriel refuses to participate in the challenge, causing Jacques to fail. Angry that he was not given the chance to at least try, Jacques becomes influenced by Hawk Moth and turns into Jackady, an extravagant villain who can hypnotize someone by throwing playing cards at them.