A rainy morning, Yumeko Jabami greets Ryota Suzui who is seen without his "pet" tag. Ryota greets her by her last name to which Yumeko responds that it won't do and tells him to use her first name as she now considers them friends.
On the way to their classroom, Ryota remarks that he's planning on paying back Yumeko for the money she gifted him to which she tells him that it isn't needed as the money was a gift.
As they walk into the classroom, the two of them see Mary standing in front of her desk that is covered in graffiti, a broken doll that resembles her, a pet tag and a knife that is stabbed into the desk.
Yumeko concerned and confused by this asks Mary Saotome if she's being bullied. This causes Mary to tell her to shut up as she thrusts her tag into Yumeko's face and coldly responds that this event is all because she lost against Yumeko in a gambling match.
Various students then tell Mary to hurry up and put on her tag or else she'll face severe consequences for not doing so, one of them being her debt increasing. As a taunt, the class begins to chant "mike" (cat) at Mary until she slips on her tag.
At lunch, Ryota attempts to placate Yumeko, by telling her that what occurred to Mary isn't her fault but simply the rules of the academy. The academy has many students from influential backgrounds, as a result of this, the school decided that skills such as bargaining, lip reading, and the ability to compete are important skill sets that are essential for the making of students who will eventually become leaders in their respective fields. Ryota further explains that those who are good at gambling inevitably become popular, whereas those who are bad at it become persecuted instead. And as a result, a ranking system among the students was set up and the "Student Council Payments" manages all gambling within school grounds. Students are capable of elevating their own individual ranking within the school by making payments to the student council.
The rankings are announced at irregular intervals. If a student is in the hundred lowest ranking students out of the 3 thousand who attend the academy then they are treated as an uncooperative student, a "pet." Male pets are regarded as "Pochi" while females are called "Mike.'
Ryota reveals that he got rid of his pet status by paying 1 million yen to the student council due to the money Yumeko gave him and proceeds to thank Yumeko again when a commotion breaks out. They see a male "pet" being mistreated by a pair of students. Ryota remarks that "pets" were never treated so harshly in the past, but conditions changed when the current student president took office 2 years prior to the start of the series. The current president challenged the former to a gambling game, with the position being the betting offer. As a result of winning the match, the current president reformed the system that was in place and saw the creation of the "pet" system and the "Student Council Payments"; this reform further increased the intensity and frequency of the harassment that those who were considered as pets faced.
Yumeko's hand is then seen to be shaking which causes Ryota to apologize for scaring her. Yumeko becoming excited expresses her joy at having transferred to the academy, that she dubs as being unusual.
Yumeko and Ryota then see a group of students who are even gambling at lunch. Ryota reveals that one of the gamblers is a first-year student and a member of the Student Council, who is also insanely rich as her father is the president of a large toy company coupled with her being good at gambling. Yumeko excitedly expresses her desire at wanting to play against her in a game to which Ryota attempts to sway her that she shouldn't.
Itsuki Sumeragi appears at the table Yumeko and Ryota are sitting at, greets her and introduces herself to her. Ryota attempts to ask her what she's doing, but she ignores him and begins to admonish Yumeko for defeating Mary at their gambling game yesterday. And tells Yumeko that's she pretty good at gambling and would like to challenge her to compare their skills. Yumeko accepts.
Back in the classroom, Mary is reflecting on how she managed to lose her social status to the current position she's in. A student spills their coffee and calls out to Mary to come and clean up the mess. Mary becomes angry at being referred to as a pet and is seen getting up from her desk and roughly grabbing cleaning supplies. As she's cleaning up, another student comes into the classroom and announces that Itsuki is having a gambling match against Yumeko. Excited students begin to run out of the classroom to head to the cafeteria.
In the cafeteria, Itsuki explains to Yumeko that they gambling game they are playing is called "Double Memory." The game utilizes two decks of cards and requires the player to match both the suit and number on the card out of the 104 card deck in order to earn a point. Itsuki places down 20 chips worth 1 million yen that are issued by the student council, for a total of 20 million yen. Itsuki asks Yumeko if she's willing to make a bet of 20 million yen to which Yumeko replies that she simply doesn't have that sum of money on her. Itsuki becomes curious and tells Yumeko that contrary to her appearance it seems she spends money quickly before asking her if she used the money to pay off an existing debt she has. This causes Yumeko to glance at Ryota who becomes visibly flustered.
Itsuki tells her that there's no need to worry as she'll lend her the 20 million yen without interest. Ryota asks her what she'll gain from lending Yumeko such a large sum of money. Itsuki tells them both that 20 million yen isn't really a big deal to her, but getting people interested in her father's company's products, especially the card deck that she is using that was created by the company is what really matters. Yumeko agrees to these terms, sensing that it's a good deal.
Itsuki is then seen advertising the card deck that was created by her family's company via a camera in the cafeteria that is seen broadcasting to other students, promoting that they're the second most popular card brand in Japan and that they feel good in a player's hand before urging other students to buy a deck.
The gambling match begins with Yumeko going first at the urging of Itsuki. Neither of them seems to find a match in the deck as Ryota wonders if the game is a little extreme as the probability of getting a match in the deck using the rules of the game is 1 out of 103 and the only way for a person to be able to find a match or win the game is for them to remember the positions of existing cards they've already flipped over which is humanely impossible. Yumeko manages to secure the first point by finding a match for the 4 of diamonds. Since she found a match, Yumeko goes again but is unable to find a match as she flips over a king of clubs and a seven of diamonds. Itsuki seems to remember seeing the king of clubs earlier and flips over its match, essentially earning a point as well. As the match progresses, Itsuki and Yumeko begin to match up many of the cards, with Yumeko finding 18 matches while Itsuki finds 22.
A student observes that neither Itsuki nor Yumeko have made any mistakes in locating the matching pairs of the cards and notes that it's odd as it seems that both of them are memorizing the location of all the cards they've flipped over. As this is said, Yumeko flips over another matching pair. The game continuously becomes evenly matched as both players are keeping pace with each other.
Itsuki admonishes Yumeko for her ability to gamble and tells her that her match is the first time she's had a game that was good. She then proceeds to produce 10 existing matches out of the remaining cards left, furthering the close gap between her and Yumeko. She flips over a pair of cards that prove not to be a match and tells Yumeko to do her best on the next pair or else she'll win the game. Yumeko fails to produce a match and Itsuki wins the game having won a majority of the 54 pairs of cards in the game, leaving Yumeko with a debt of 20 million yen.
Yumeko begs Itsuki for another match as she can't afford to pay her the 20 million debt that she now owes, but has the same ability as her to memorize the location of the cards. She exclaims that she doesn't understand why she lost. Itsuki grasps Yumeko's hands in her own as a blush overcomes her face, she tells Yumeko that nails are a hobby of hers as she forcefully presses Yumeko's nails with her fingers causing Yumeko to flinch in pain. Itsuki moves to a briefcase that is placed on one of the couches in the cafeteria and opens it up, revealing a collection of painted nails that are protected by glass. Itsuki reveals that her nail collection are all human nails that she won via gambling matches as no one is willing to sell her real human nails despite the vast amounts of money she possesses. Itsuki says that she's willing to bet another 20 million yen, but that if Yumeko loses she desires her nails. Yumeko agrees to these conditions much to the concern of Ryota, whom she tells that there isn't any way for her to gain 20 million yen to pay off her debt unless Ryota is willing to pay for her. Ryota tells her that doing so is impossible for him.
Itsuki produces two brand new decks of cards for the game. Itsuki reveals in a monologue that out of the two decks, one is a deck that she specially designed; this deck has a temperature mechanism in which once the cards reach a certain temperature a small mark appears on the back of the cards that differentiate them from a normal brand. She has a circuit in her bag that allows the cards to keep warm, allowing the marks to remain upwards of 2 to 3 minutes, this requires her to quickly find and remember the location of the cards that have the marks on their back. As she lavishes at the ease in which she'll yet again win the match, Yumeko produces a card match upon her first turn. Yumeko then tells Itsuki that she heard her family runs a toy company and that their net worth must be pretty substantial even though it seems she's willing to only bet low amounts of money that will have no effect on her family's worth. Yumeko gives her advice, telling her that before she's willing to bet her family's business she has to be prepared. This cryptic advice confuses Itsuki, who immediately realizes that Yumeko may be aware of her trickery. She realizes that if her family's company were ousted for producing cards that could be used to cheat it would damage the image of their business. As Itsuki becomes concerned that Yumeko knows the full extent of her trickery, Yumeko flips over 54 matching pair of cards in a single turn, effectively winning the game.
Itsuki shocked, asks how this game to be. Yumeko tells her that if she were just a little bit more careful then she wouldn't have caught onto how Itsuki was rigging the game in her favor. Yumeko reveals that the image on the back of the rigged cards are exactly the same as the previous rigged deck that they played with last and if Itsuki had simply prepared a new and different deck with a different mark then she wouldn't have caught on to the deceit. Itsuki is shocked at this as she realizes that Yumeko had to have caught on to this deception from the very beginning of the previous game and memorized the locations of the marked cards while playing the game as she pretended to not know the locations of the marked cards. And that Yumeko was simply acting upset after the ending of their first match in order to win their second match.
Itsuki is in disbelief at both the results and Yumeko's inhuman ability to seemingly remember the patterns on the marked cards and make all the matches in the span of a few minutes that it would take for the marks to disappear. Itsuki suddenly becomes aware that Yumeko is beside her and takes a few steps back as Yumeko asks what card game they shall play next. Itsuki nervously refuses another game as Yumeko taunts her and then suggests that they both put their nails up for bets in the next game, suggesting that they have news terms this time, with if one of them loses they must "have all the nails on their hands and feet ripped off." Itsuki becomes disturbed at this as Yumeko further tells her that having one's nails ripped off is extremely painful and makes everyday life difficult. Yumeko, becoming extremely excited at the idea of such a high-risk bet, tells her that the idea of having such a gambling match gets her excited.
Telling a trembling Itsuki, Yumeko announces that such a gamble is a once in a lifetime opportunity and that even though Itsuki confidently cheated in a game and still managed to lose, she wonders what will happen to Itsuki if she doesn't rid herself of such shame. She presses Itsuki to agree to the terms of their match causing Itsuki to scream before collapsing.
Itsuki tearfully tells Yumeko that she can't commit to such terms in a gambling game, but asks her to forgive her. Yumeko stares blankly at her as Itsuki again asks for forgiveness to which Ryota tells her that she's attempting to ask Yumeko to accept her apology. Yumeko responds that it's boring which earns a look of shock from him.
In the student council room, a member reports the outcome of the match which stirs up the other members of the council who are surprised that a transfer student could have been Itsuki at her own game. To which another member responds that Itsuki only was a member of the student council because of her large payments to them. The members ask the president what it is she plans to do about Yumeko. To which the president tells them that they'll simply meddle with her a little bit.
In the hallways, Ryota, surprised, asks Yumeko if she's really going to gamble more after school. To which Yumeko responds that he was the one who told her that lots of places after school transform into gambling dens and then asks him where they are and suggests that the two of them go together.
The two of them go to the Traditional Culture Club room and are told that there's a public match occurring at the moment. In the club room, an exhausted Mary is seen being told that she now has an accrued debt amount of 49.6 million yen. Mary's opponent tells her that she'll be in contact with her later to arrange a payment plan for her current debt, before asking that someone escorts her out of the room as she no longer has any business with her before Mary begins to scream in anguish.