You remember that time that Chloe sent Sabrina to the Dupain-Chengs' house to sneak into Marinette's room and find something which Chloe could use to embarrass Mari in front of the whole school? And Sabrina tried to steal Marinette's diary, but Mari had just invented her own secret security system to protect her diary, and then Sabrina got her hand locked inside a security box containing the diary?
Did you ever wonder what was IN that diary? Like what Marinette thinks about Cat Noir, what she thinks about Adrien, her best friend Alya, and her other friends? She writes all this stuff down later, after she's defeated Hawk Moth's latest threat and saved the day (and after she's done her homework).
I don't know if everyone's seen this: