01. "A Girl Named Yumeko Jabami" (蛇喰夢子という女, "Jabami Yumeko to iu onna")
02. "A Boring Girl"
03. "The Slit-Eyed Girl"
04. "The Crazed Girls of Hyakkaou Academy"
New transfer student Yumeko Jabami is guided about the school by house pet classmate Ryōta Suzui. She is challenged by Mary Saotome to a jankenpon (rock-paper-scissors)-styled gambling game. At first, Yumeko wins and loses a few rounds, and ends up out of chips, but she figures out how Mary has fixed the game in her favor, and when the stakes are raised, soundly beats her. She frees Suzui of his debt and becomes friends with him, revealing she has a maniacal passion for gambling. Yumeko is then challenged by card expert Itsuki Sumeragi to a game of two-deck Concentration. Although Yumeko loses the first game, when Itsuki mentions she can wager her own fingernails, Yumeko accepts the challenge and then defeats her after seeing through her card marking. Yumeko is challenged by Traditional Culture Research Club president Yuriko Nishinotōin to a roulette-like game called "Life or Death". Although she determines how Nishinotōin was able to scam so many people in the game, her final big wager does not go the way she expected, and she becomes a pet, after which she suspects the student council has taken extra measures to fix the game.
With 310 million yen in debt, Yumeko is now a pet. Yumeko and Mary receive Life Schedules in which all their future career and family choices will be pre-determined if they are unable to clear their debts. Yumeko meets bullying student Jun Kiwatari who tries to use his position as master to make her undress, but a crazed schoolmate named Midari Ikishima who carries a pistol for Russian roulette intervenes. Yumeko and Mary join a debt reassignment contest in which they can assume the lesser debts of one of the other players depending on how they finish, and of which the winner would be cleared of their debt. They are grouped with a quiet girl named Tsubomi Nanami, and Kiwatari who borrowed money to join in to humiliate the girls and is also Tsubomi's master. During the game of Indian poker, Yumeko reveals how Kiwatari has been getting signs from Tsubomi on her cards, but that she and Mary have also been giving signs to each other, and it now comes down to who can bluff the best. They convince Tsubomi to not listen to Kiwatari's advice and to free herself from thinking she is just to be a pet. Although Kiwatari thinks he has won the game, he learns that he placed last overall, as Yumeko and Mary had switched the values of their chips.
Although Mary has paid off her debt and is no longer a pet, she gives the rest of their winnings to Yumeko, who does not plan to pay off her debt yet so that she could challenge the student council president Kirari. Student council member Midari abducts Yumeko and Ryota, and challenges Yumeko to an ESP Russian roulette game: Each player must guess a sequence of five Zener cards; the one who gets the most correct gets to choose one of two revolvers in a box which were preloaded by the players with 0-6 bullets in its chambers, and shoot the other player a number of times based on the difference in their scores. Yumeko agrees to play with the conditions that they play only three rounds, that Ryota be the dealer, and that the loser must pay 1 billion yen, while Midari adds the condition that if the shooter intentionally misses, the other player can then shoot back. In the first round, Midari wins by 1 guess but picks and shoots Yumeko's unloaded revolver. In the second round, Yumeko wins by guessing correctly that Ryota would choose her previous guess as the sequence, but she again picks her unloaded revolver because she can determine its difference in weight. Midari reveals how she lost her eye when she challenged and lost to Kirari. In the third round, Midari and Yumeko each load two chambers, but Yumeko realizes the camera at Ryota's room is reversed, based on which hand he uses on the cards. But instead of going for a win over Midari, which would have guaranteed that Midari would be shot, she intentionally guesses all wrong, leading to a draw. Meanwhile, Mary is recruited by Kirari to join the student council, but upon learning how Kirari has viewed the house pets, she declines.