Discovery of the Secret!? is the fourth episode of the Midori Days anime.
Midori is seen gleefully eating an ice cream cone. Seiji asks her what she's so happy about. She says in reply it's a dream for her to eat her favorite flavor in an extra large size. Seiji says he wishes it was a dream. Midori adds she's happy to eat her ice cream with him. Seiji blushes and tells her to eat.
Seiji is seen licking his fingers and saying to himself how he should build an ice cream stand in his house. He sneezes, and tells Midori it's time to go. However, his sneeze made him spill Midori's ice cream all over her. He apologizes.
The two are seen going down a road. Midori asks Seiji if he really promised her to get her new clothes. He says yes, and she becomes excited. He tells her he was the one who got ice cream all over her only outfit. Midori, excited, tells him to take her shopping now.
Seiji takes Midori to an anime shop that's full of clothes just her size. It's also filled with dolls and doll otakus. Midori is delighted with the shop's selection, while Seiji isn't delighted to be there. He realizes if he's seen, he'll be the laughing stock of the school. He tells Midori to hurry up, but she's trying on a wedding dress and pretending to marry Seiji. Seiji yells at her, and every customer looks at him. He manages to scare them off to their own business. He says he doesn't want to be here, but to himself. He hears Midori say how he doesn't look well, and he sees she's dressed as a nurse. She sticks a thermometer in his mouth, which he spits out with annoyance. She apologizes, and he looks at the tag behind her back. It's thirty-five dollars, and he's shocked by how much the outfits cost. He sighs, and thinks how money'll be tight. Midori hands him a small purple cloth, and he pulls it, spinning her. She's revealed to be wearing a kimono. Just as he's about to yell at her again, someone recognizes him.
Seiji doesn't recognize Shuichi Takamizwa until he tells him he's his classmate. He also says that he's surprised Seiji has the same hobby as him. Seiji tries to deny his "hobby," but Shuichi tells him he doesn't have to be shy about it, since they're "friends." Behind Seiji's back, Midori is trying on a flight attendant outfit.
Shuichi says he keeps his doll hobby secret and that he has his reputation to consider. Seiji thinks he's got a lot more than his reputation to consider, but then thinks he'll tell everyone in school he saw him there. He plays along with Shuichi. Shuichi agrees to keep it secret, and tells Seiji that he loves dolls because real girls are never how you want them to be. He says he thinks small, cute, sweet and innocent dolls are much better. Seiji realizes he's describing Midori, who is trying on a devil costume.
A scene showing Ultra-Marin, Seiji and Shuichi shows Marin asking the two what their ideal type was. Shuichi describes his ideal type, and his screen shows Midori.
Seiji realizes Shuichi can't find out about Midori. Midori gets Seiji's attention and she's now wearing a police woman outfit. She teasingly says his heart is officially under arrest, but he covers her mouth to keep her quiet. However, Shuichi becomes curious to what he's hiding. To keep him quiet, Seiji knocks him out.
The next day in school, Seiji thinks about the previous day's encounter when his teacher asks him an answer to a problem. Ayase gives him the answer to the problem and he thanks her, but she tells him don't mention it. Midori then gets his attention by dragging an anime book out of his desk. Seiji opens the book and finds a note from Shuichi, who gives him a thumbs-up.
At lunch, Ayase gives Seiji lunch, telling him she made too much. Seiji complements her on the egg and asks her to be his girlfriend. However, Ayase is merely daydreaming about it. When the teacher says it's time for class to end, she eagerly pulls out her lunch to give to Seiji, only to find he left.
Seiji rereads the note from Shuichi, which says they need to talk about his right hand. Seiji thinks he's threatening him and goes up to the roof. Midori asks him if he's going to do anything rash, which he replies with a smirk he won't do anything too rash, worrying Midori.
Shuichi is seen standing about thirty feet from Seiji, who has Midori hidden behind his back. Shuichi starts to walk towards Seiji and says he always has his right hand hidden. Seiji thinks he knows about Midori.
However, Shuichi throws himself at Seiji's feet and begs him to let him see his "puppet." He holds up an Ultra-Marin puppet and says he has his own as well. Midori tells Seiji she doesn't think Shuichi has figured out the secret and Seiji agrees.
Shuichi and "Marin" beg Seiji to let "them" see Midori. Midori devises a plan and has Seiji follow her lead. Seiji agrees to let Shuichi see Midori, much to his joy.
He shows him as Midori stays still like an uncontrolled puppet. Shuichi becomes fascinated with her, and asks him where Seiji got her. He lies, saying his dad brought her overseas. Shuichi is excited she's a "foreign model," just as Midori sneezes.
Seiji snaps at Midori for sneezing, as Shuichi is ranting on how she sneezed. "Marin" says "she" can't do that. The two realize that Shuichi still thinks she's a puppet and they try to leave. However, Shuichi asks what company made her, but Seiji lies and says he promised to keep it a secret. He tries asking again, but Seiji flees and slams a door in his face.
Early the next morning, a delivery truck delivers a package. Midori opens it and finds a dress just her size. Seiji thinks Shuichi is up to something.
Unknown to them, Shuichi is watching them (with the cat from every episode on his head). His plan is when Seiji changes his "puppet's" dress, he'll see the company logo on her back.
Seiji thinks the dress looks safe enough, and Midori eagerly begins to change, but not before she asks Seiji to close the curtain. Seiji grants her request and Shuichi thinks there's something on his "puppet" that he doesn't want him to see.
Shuichi follows Seiji and Midori throughout the day and watches Midori's expresses change, thinking it's an amazing gimmick. He follows them to a meat shop, where he sees Midori speak out in favor of not buying expensive tuna. His glasses break and he decides he must have Midori.
Seiji, on the way home, senses someone following them and hides. Midori takes a look herself and is startled by Shuichi. Seiji becomes angry and Shuichi apologizes, saying he had to know where Seiji's "puppet" came from. Seiji is annoyed, but Shuichi tricks him into thinking there's an 100 dollar bill (about 8000 yen) on the sidewalk. He lifts Midori's dress and sees she's not a puppet.
Seiji becomes angry at him for finding Midori's secret. Shuichi thinks of the situation as crazy and expresses his jealousy of Seiji. He tells Midori she's just his type, making her blush and thank him. Seiji tells her it's not the time as Shuichi asks how it happened. Seiji tells him he doesn't know and threatens him. Shuichi recognizes the threat. Seiji asks him what he wants. Shuichi tells him he wanted Midori to be his puppet, but is wasn't possible. He tells Seiji he'll make him his puppet instead. He leaves.
Seiji and Midori go to the place Shuichi asked to meet him. Shuichi arrives with two bags.
Midori is then seen in a maid's outfit. Shuichi is seen taking pictures of her and saying how cute she is. Seiji, annoyed with having to lie down for Midori's pictures, leaps up and tells him to stop. Shuichi tells him he can't move too much. Midori then asks if Shuichi made the outfit himself, which he replies he did and asks her if she likes it. She says she does, and he gives it to her. She promises to take good care of it, much to his joy.
Shuichi admits he's fallen in love with Midori and asks her to be his girlfriend. Midori declines, and says as nice as he is, she loves Seiji and only Seiji, but they can still be friends. Shuichi accepts her friendship and tries to use it to get her to try on a bikini. Seiji becomes angry with him and angrily (yet unintentionally) knocks him out.
The next day, Shuichi thanks Seiji for getting him home. Seiji's martial arts have appeared to turned him normal, as he greets two girls in a normal fashion.
That night, Midori is recounting the previous day's adventure. She grins and says she's happy.