A boy and his Arnval Shinki, nicknamed Ann, move to his hometown in Japan to start high school. As Ann helps unpack her master's things whilst he is at school, his other Shinki, Ines and Lene, help her out and discover a box labelled 'Important'. After tidying up the place, they inadvertently send a knife flying into the 'Important' box and decide to check the contents, finding a love letter inside. When the wind suddenly blows the letter away, the Shinki chase after it as it knocks over another Shinki named Kurara, who takes it as a challenge and forces them into a battlefield. When the letter becomes slightly damaged as a result of the battle, Ann becomes angry and overhwelms Kurara in battle before heading back home. After their master returns from school, Ann apologises for what happened to the letter, which is revealed to contain a visualizer containing a house he made for Ann when he was little. Later, they discover another box containing a Strarf model Shinki.
The Strarf unit, which was a surprise gift from the master's father, is named Hina. As Hina becomes wary of a presence nearby, she becomes downhearted when she learns her master isn't all that into battling. Later that night, Ann suggests to Ines and Lene that they prepare a feast for their master's high school entrance gift. However, things start to get messy when a pair of cat-type Shinki start interfering. Things soon get suspenseful when the fuse shorts out and the Shinki find themselves attacked by a live octopus. Luckily, Hina comes in to save the day, stopping the octopus and fighting back the perpetrators whilst managing to get dinner ready in the process.
Kurara shows up at the Shinkis' home asking for their assistance in catching a thief that has been stealing Shinki units. As Hina decides to accompany her, they tail a suspicious person to a Shinki store. Meanwhile, the master is taken in for questioning, prompting Ann, Ines and Lene to set off in order to clear his name. Noticing he is being watched, the suspicious person activates two Shinki, Ach and Yda, against Hina and Kurara. As Ines and Lene chase after the thief, Ann goes to assist Hina in fighting Ach and Yda. The battle is soon put to a halt when Ach and Yda learn that their master is the true thief who manipulated them into fighting for him. As Ach and Yda head off on a journey to find a more suitable master, the thief is brought to justice and the master's charges are dropped.
Seeking a cool place to relax, the Shinkis inexplicably end up huddling around a room full of candles, telling horror stories in hopes of scaring the heat away. As the weather suddenly shifts to a downpour, the Shinkis go out searching for their master and encounter something not of this world...or so they think.