A young girl walks into her older brother's room one day hearing a strange noise coming inside of it. All too familiar with his chuuni antics, the dutiful little sister expected to see a practice run for his hijinxes gone wrong when she went in but is baffled to find a futuristic looking smartphone sucking her brother into a vortex of light.
When the dust settles, she finds her brother trapped inside the phone and he instructs her to take the stack of trading cards on his table and go to a point on the map in his phone. He explains that he was tricked into his current situation he needs her help to get free. Once she gets to the marked location, the girl is forced into a magical card game that makes use of the cards she brought with her.
Having followed her brother around sometimes on his overly excited card games, she prepares to bring out her own skills in what she expected to be his favorite TCG come to life. A twist creeps up on her however when it turns out that this magical dueling system that imbues cards with powers over reality is not restricted to a particular game but incorporates several different ones instead mixing and matching rules and effects depending on the cards being played.
Just barely getting by her embarrassingly chuuni first game with her brother's advice, she acquires her first personal magic card that bestows cryptic advice on whatever question she's got on her mind at the time. Her brother explains that she needs to track down and win a particular set of cards to set him free but she needs to build her own personal deck and fighting style to accomplish that goal.
Torn between wanting to live her adolescence free of lifelong shames that would haunt her future self and saving her dear brother, she reluctantly dons a costumed persona to participate in these chuuni card duels and makes her way into their wondrous albeit embarrassing subculture.