The series begins with an episode of Serious Sentai Gachiranger, in which the evil Gedol Empire try to eliminate children from the Saitama District so it will be easier to invade, using a monster named Crabron to turn everyone into a pile of bubbles. The Gachirangers hear about this and set into action, but seem to screw up at every opportunity. The Gachirangers later confront Crabron, culminating in a giant robot vs. monster duel. Upon defeating him, all the children return back to normal. Meanwhile, Hitoha is frustrated that the episode was filled with a lot of stupidity compared to the usual.
While protecting a photo she took with a Gachitanger, Hitoha inadvertently sparks a rivalry between Sakiko and the Hopeless Squad. Mitsuba desires an exercise machine and uses Futaba as a replacement. Yabe tries to work up the courage to ask Aiko to a Christmas party, but she loses her glasses and starts wrapping everyone wearing red clothes in bandages. Yabe ends up spending Christmas alone while Yuki holds a mixer party, though no one seems to go with her ideal picture of the King game. With Souijiro coming home late, the girls get Yabe to fill in as Santa for Futaba, though she ends up gripping tightly on his clothes, putting Yabe in a rough situation when Soujiro comes home.
Futaba decides to draw an erotic manga for Yuudai, roping in Shinya to help her learn to draw underwear on her figures. The school practices a fire drill, with Yabe's imagination getting the better of him. During a sports event, Soujiro ends up losing his clothes in preparation for a three-legged race with Hitoha, landing him in an awkward predicament. Yuudai invents a new move to unhook girl's bras, which leads to various misunderstandings while trying to find someone who actually wears a bra. While over at the Marui house, Mitsuba's friends find an old photo of Soujiro when he was more handsome, with Mitsuba claiming him to be her boyfriend.
Futaba makes a handmade waterslide which ends up ripping the swimsuits Mitsuba, Miku, Yuki and Miyashita, who are all too embarrassed to admit it first. When Mitsuba nearly drowns after getting a cramp, the others risk embarrassment to save her only to get mocked by her afterwards. Later, Miku shows faked photos of a supposed ghost following Mitsuba around to freak her out, which backfires when Sakiko and Hitoha get involved. At a festival, Futaba notices Shinya looking depressed and tries to cheer him up with underwear. She later learns he has a fever and forced himself to come so he could hang out with everyone. Yuudai comes up with yet another perverted maneuver, but is once again foiled. Later at the city pool, Hitoha almost drowns when her float bursts, but is rescued by Futaba.
When the Hopeless Squad hears a rumour about writing your crush on an eraser, one of the members, Shiori Itō, writes her name on Shinya's eraser, but Airi figures her out. On Parent's Day, Marina arrives early and decides to survey the class, overhearing that Miku is jealous that she always pay attention to the weird kids instead of her. On Valentine's Day, Mitsuba tries to make Miku jealous of her homemade chocolate, but it backfires when Miku's pricey chocolates taste delicious. Mitsuba, Yuki and Miyashita try to get together to secretly make Miku some cookies for White Day, though their attempts to keep it a secret from her leads Miku to think they hate her. Sakiko meanwhile thinks Miku is dead and grows attached to her, not taking kindly to the White Day gift when it's revealed otherwise. As the girls take their physical exams, the boys decide to hold rankings for the girls, coming to the agreement that they'd be good if they did not talk, which comes to pass when they receive their results.
In the minutes leading up to the New Year, Mitsuba occupies the toilet, leaving Hitoha and Futaba desperate after eating some bad oranges. As revenge, Hitoha decides to shut her in, leading her to get stuck in the window when she tries to escape. On Valentine's Day, Soujirou becomes furious that one of the girls is giving someone chocolate and swears revenge on whoever's receiving it. When he impulsively enters the school for that reason, he ends up meeting Shinya who has his own Valentine's woes and helps him to evade the Hopeless Squad. At a parent-teacher meeting, Shinya's mother, Akari, shows concerns about Shinya being stalked by the Hopeless Squad, clashing with different opinions from Yabe and some of the other parents. Ironically, Airi's older brother is a policeman, with Yabe having to act quickly to keep Airi from knowing she's the stalker in question. As snow appears on the playyard, the class becomes fearful when Mitsuba starts throwing snowballs made from snow that Yuudai had peed on. On April's Fools Day, Hitoha becomes so overly cautious about being pranked that she decides not to trust anyone. She ignores Yabe's and walk straight into a post and falls unconscious, only to then be pranked the moment she wakes up.
With her dad overseas during Christmas, Miku tries to get her friends to dress as Santa to make Ryuuta believe he exists. Noticing Hitoha's habit of hiding under Yabe's desk, Miyashita tries to utilise that so she can become friends with her. Later, class 6-1 teacher Kaieda fills in for Yabe's class and comes into trouble whilst dealing with Futaba, who is using written notes as she has lost her voice. Oh a hot day, Mitsuba goes to great lengths to get the air conditioner working, only to foiled by her sweat and weight. With the class assigned to pool cleanup, the boys and girls have a human curling contest to see who has to clean up.
Hitoha is nervous about meeting the Gachirangers' Gachi Pink at an autograph session, but ends up catching a cold on the day. She ends up going anyway, though when she does not want to be seen by Yabe, she ends up hiding under Gachi Pink's table, where she manages to thwart a guy taking upskirt photos. In order to win a relay race against the other sixth graders, Yuudai and Hitoha come up with a plan to take advantage of each runner's traits. A sweet potato salesman with an eye for telling a woman's age is put to the test when Hitoha challenges him to guess how old she is. Later, Soujirou remembers when his daughters were younger and more well behaved.