Adrien is in despair as he has no one to write a love note to until Nino suggested he write it to a friend, so he chose Ladybug. The note read: "Your eyes are blueberry blue, so is your hair too. I wonder who's behind, the mask that hides. You're my valentines true, your friend Adrien to you." Unfortunately, after writing the note. He realized he had no way of delivering the note without spoiling his secret identity. So, he ended up throwing out the note. Only for Marinette to dig it out of the garbage and be convinced it's for her, later that day Adrien finally realized how to get his feelings across when he saw a picture from Chloe. It was a humiliated Kim who was trying to confess to her, Adrien immediately knew that Kim was going to get akumatized over this and so transformed.
Finding Dark Cupid (Kim's Akumatized form, dubbed by the news) about to hit Chloe with one of his arrows Ladybug lassoed the arrow in midair and destroyed it. While Cat Noire attacked Dark Cupid who was firing arrows at her in response, which Cat Noire destroyed with her staff while she got closer. This is when Ladybug called for a "Lucky Charm!" Which produced a jar of honey, she immediately threw the jar at Cat Noire who hit it at Dark Cupid. Which hit him and broke, covering him in honey. He tried to launch another arrow but found it stuck to his hand. That's when Cat Noire struck him with her staff, knocking him out. She then cataclysmed the Akuma object.
Later that day Adrien received an answer to his valentine letter which read: "Your eyes are green, I like you teen. Your hair is like the sun, I wish I can reveal my feelings and be done." Adrien said "Someone answered my letter but it isn't signed. (then a ladybug flew onto the letter) That's it, it must be from Ladybug. I just know it."