In the mood for some good old-fashioned gambling fun? Head on over to Howard Resort, a famous entertainment destination that's home to the voluptuous, and said to be lucky, dealer Rio. Alongside other casino dealers and staff such as the clumsy Anya and bunnygirl Elle and Ille, Rio helps tend to the whims of the resort's customers. With Mr. Howard constantly forcing her to wear more and more skimpy outfits and even a ghost tinkering with the hotel's plans, it won't be easy! Especially since Rio is the holder of a very special card known as a Gate – a card that other dealers of the world want desperately to acquire, as only then will they earn the title of Most Valuable Card Dealer...
Mint Clark goes on holiday with her grandfather to the Howard Resort. While her grandfather enjoys the casino, Mint explores the resort and eventually meets the casino's most famous dealer, Rio Rollins Tachibana. As Rio and Mint go for a walk, they are approached by a man and his henchmen who attempt to take Mint's teddy bear, Choco, but Rio fights them off. After Mint bears witness to the good fortune that Rio brings to her customers in the casino, they are once again approached by the man, Orlin Dunhill, who decides to play a game of closed poker against Rio for Mint's bear. Orlin makes a gamble to get four Queens though, but thanks to Rio's ability to communicate with her cards, he loses out to Rio who wins with a pair of twos. Afterward, Orlin explains he had been collecting rare stuffed animals to fulfill his sorrow from being dumped at the altar, only to then find that Mint's bear isn't a Belgium original.
Rio is asked to train a new recruit named Anya Helsing to be a dealer. Anya is quick to point out that Rio owns a "Gate", one of thirteen rare cards that are wagered in battles with other "Gate Holders" to determine the strongest dealer in the world. Although her dealer skills are quite good, she is rather clumsy, causing some chaos in the casino. Meanwhile, a Gate Holder named Elvis comes to challenge Rio to a Gate Battle set on a giant roulette table. Although Elvis has confidence is his prediction of numbers, he ultimately loses to Rio when a bandage he was wearing on his finger comes off while throwing the ball, allowing Rio to guess correctly and win his gate.
Howard takes Rio to a supposedly haunted castle which used to feature a casino owned by a woman named Misery, with plans to refurbish it as part of the Howard Resort. During the stormy night, lightning strikes the castle, which frees the ghost of Misery, who goes to the Howard Casino and spreads bad luck. She attempts to try and win the casino in a game with Anya, but instead falls victim to her clumsiness and freaks out. Misery later attacks a chicken wing delivery van, and when employees El and Il Adams discover her, they are attacked too. The next night, Rio gets a call from fellow employee Rosa Canyon, who had been possessed by Misery's spirit. Upon returning to the castle, Rio finds Misery, who challenges her to a game for the Howard Resort, having already taken control of El and Il and kidnapped Howard. The game involves betting to see if the dice will land on an odd or even number, but Rio is troubled by Misery's illusions. However, they are dispelled when Misery eats a super-spicy chicken wing she got from a mysterious delivery girl, allowing Rio to win the game and free Rosa. As Rio chases after the delivery girl, she reveals herself as her childhood friend, Rina, who has also been recruited by Howard.
Rio takes Rina on a tour of the resort, introducing to the other employees. Rina also notes that Rio is a Gate Holder, mentioning how Rio's mother, Risa Rollins, once owned all 13 Gates. Their date is cut short when a man named Bull Hard takes Anya hostage. He challenges Rio to a vending machine lottery game, but is easily defeated when he tries challenging her to a drinking contest with a hot drink. The next day Rina begins work as a dealer, and her aura seems to make customers weary. Howard then decides to put both Rio and Rina in a game of clay pigeon shooting where players lose items of clothing if they miss; this task made tougher by the wind created by the excited customers. When Rio is at a disadvantage due to the appearance of seagulls during her turn, Rina throws her final turn, ending the match in a draw. Later that night, Howard gets ready for an opportunity with a robotic girl named Linda while Bull is seen talking to a mysterious woman interested in Rio's powers.
Howard sends Rio, Rina and Anya to work at the Sky Resort casino situated at the top of a mountain. Once there, they meet the resort's dealer, Linda, who gives them a tour of the facilities. She is soon revealed to be a robot, and the resort's main terminal. The resort opens the next day, floating itself and its customers high into the sky. Just then, a boy reprograms Linda to challenge Rio to a Gate Battle in which they must both race each other down a giant water slide. The match is a close call throughout, with Linda manipulating the course to catch up; however, Rio wins when Linda loses her head. After the boy relinquishes control of Linda, the mastermind, Cartia Goltschmitt, approaches Howard.
Howard informs Rio, Rina and the others that Cartia had challenged Rio to a Gate Battle against someone named Jack with Howard Resort on the line. Mint asks Linda to draw the culprit who manipulated her data, which leads her to suspect Carlos Tanaka, an Afro samurai with terrible luck, as the culprit. Meanwhile, Rio, who has been getting a lot of silent calls lately, is shocked to find Misery's portrait in her room. As Mint follows Carlos, her bear almost falls off the resort but is rescued by a boy with telekinetic powers who is revealed to be Rio's opponent, Jack Mighty. Jack challenges Rio to a game of Space Pinball for the Gate. Before the match, Mint talks with Jack, who reveals he used to be in a bomb disposal family before he was taken in by Cartia, who just wants to use him for his powers, saying he'll be free if he wins against Rio. During the match, Jack gets the advantage due to his powers but is soon overwhelmed when holographic sharks appear. Rio helps him escape the sharks, but he attacks her ship. However, Rio exits her ship and gets her own back to take the lead and win the match. However, when Carlos accidentally knocks Linda's head off, it activates a more dangerous phase which puts Mint in danger, so Rio and Jack work together to help her and stop the malfunction.
Only two hours after the battle against Jack, Howard reveals another Gate Battle will take place against a woman named Queen despite Rio being exhausted from the last battle. The battle sees Rio and Queen flying up a wind tunnel while shooting targets. Meanwhile, the others learn that Cartia had hacked the Sky Resort's computers, preventing them from landing, with Jack revealing Cartia won't release it until Rio loses. Queen attempts to cheat to get an advantage over Rio, but another incident involving Linda's head falling off again turns it around for Rio. With Rio physically exhausted from her battle, Rina looks after her and takes her to a bathhouse, where Rio feels someone is watching her. Afterwards, Rina is taken hostage while Rio is locked in the hologram room, facing against dangerous physical holograms which cause her to faint. Rio barely has time to recover from this before Cartia arranges yet another Gate Battle against a man named King with Rina held captive.
In order to save Rina, Rio has to guess which hand King will have a coin in after he flips it, not made easy by Rio's fatigue and King's history as a con artist. However, Rio sees through his strategy of destroying the coin, and picks one a hand that's guaranteed to be empty. However, Rina disappears again. Meanwhile, Jack hacks into the systems using a videogame-like controller to try to take control of the flight system. Despite being told to rest, Rio goes to search for Rina, and is shocked to discover she is a Gate Holder working under Cartia, who is revealed to be her aunt. Rina claims to be seeking revenge against Rio's mother, Risa Rollins, for allegedly seducing her father and convincing him to leave her weak mother, who fell into a coma out of shock, revealing they are blood-related. A Gate Battle takes place in a game of poker, where Rina's superior Roll Ruler abilities win against Rio, who falls unconscious.
As a result of Rio's defeat, Cartia takes control of the Howard Resort and remakes it into the Goltschmidt Kingdom, leaving Howard with only the Sky Resort which is left floating in the sea. Upon returning from a movie shoot, Rosa is approached by the fortune teller Dana, who reveals that the Casino Guild is working with Cartia, who is trying to locate the last two Gate Holders for Rina to battle. Meanwhile, Orlin and Elvis, who haven't enjoyed gambling as much following Cartia's takeover, come to challenge Rio in a game in which the loser must leave the casino forever. Rio wins the games, but then Misery takes over Rosa again, revealing Jack had used his powers to help Rio win and mentioning Cartia plans to tear down Misery Castle before she leaves. Howard later calls Rio over to help him sabotage the Goltschmidt Resort but they are caught. Rio rejects an offer from Cartia to be a general manager and leaves, warning Rina on her way out. When she loses track of Howard, she encounters a Joker and she, along with Dana, are transported inside someone else's Roll Ruler.
Rio and Dana follow her ferret, Chip, to a casino where the mysterious Joker is holding Howard captive. She challenges Rio to a game of blackjack in order for her to escape her Roll Ruler, which forces them into a draw at every play. As Rio realizes her compassionate ways made her ignorant of Rina's true feelings, she ends up losing a hand. However, after Mint and Jack arrive, Rio manages to win a hand, to which Joker presents with a Gate and frees her from her Roll Ruler, deeming her worthy of challenging Rina again. Meanwhile, Rina becomes curious of Cartia's motives for getting all the Gates. Later, Rio learns Dana is the final Gate Holder, who challenges her to cross a trap filled bridge in order to win her gate. Using her Roll Ruler, Rio figures out the positions of the traps and how to turn them against her, allowing her to win. As Howard converses with the true identity of Joker, Rio and her friends head to Goltschmidt Kingdom in order to confront Rina again.
Rio and her friends enter Goltschmidt Kingdom and start winning big in the various attractions in order to be able to buy back the resort. However, Mint's bear Choco goes missing, so Rio goes with her back to the casino to search for it. Meanwhile, Howard talks to the true Joker, Risa Rollins, who reveals she hid herself from the Casino Guild to protect Rio. The next day, everyone had lost their winnings thanks to a pair of twins named Yang-Yang and An-An who challenge Rio and friends to an explosive game of Concentration. With the score tied, Rio shares her aura with Mint, who is able to find the final pair and win the match. Just then, a wayward explosion hits Mint, but she is rescued by An-An, who is revealed to be Choco, who Yang-Yang had taken to make into a friend to play with. Rina then appears and Rio challenges her to a Gate Battle.
After publicly announcing the terms of the Gate Battle, Howard takes Rio and the others on vacation to an island town to relax before the battle. While looking for Rio, Mint meets an old man, who explains Rio used to come to this place with Rina and Risa when she was young. Meanwhile, Rio visits her old vacation home where she almost discovers Rina before learning that Mint had gotten lost trying to find her. Mint is found by Rina, who uses her Roll Ruler to protect her from a bear. As Rina returns Mint to Howard, he tries to explain that Cartia is lying to her, though she says that even if it was a lie, it has made her stronger. On the night of the Gate Battle, which is to be a 15-game match of poker, Joker takes Elvis' place as the dealer. Rina wins the first two games using her Roll Ruler, but Rio hints she has a trick up her sleeve.
Rina gains a huge advantage over Rio, managing to lead 14-0. However, Rio soon starts to make her comeback, and as the two put their Roll Rulers to the test, the Gates start to glow. With the game tied 14–14, Rio takes Rina and Joker into her Roll Ruler world, where secrets cannot be hidden. Joker reveals her identity as Risa, revealing that she and Rina's father had to leave to protect Rio and Rina from the Casino Guild, who wanted to use them for evil, while Rina is shown her surpressed memories, and her resolve for fighting is shown. Upon returning to the real world, Rio wins due to her Royal Straight Flush and is declared the new MVCD. The Gates then activate to form the Rainbow Gate which Rio enters. However, the casino is attacked by Casino Guild radicals who try to capture Rio, but are stopped by Carlos, who reveals himself to be a member of the ICDG Police, revealing the radicals' intentions to use the MVCD for evil and convincing them to retreat. Meanwhile, Rio's ultimate Roll Ruler spreads good fortune throughout the entire world, giving everyone good luck. As life returns to normal, some mysterious figures appear on the roof of Howard Resort.