It's the first day back after the winter break and Seki is still stuck in holiday-mode as he lounges at his desk, wearing flip-flops and sunglasses, and putting sun-cream on. He is so relaxed that he doesn't put any effort into his usual antics, so Rumi assumes that he is trying to experience a winter sun holiday that he never git this season. However, he did and he has a photo to prove it, so Rumi decides to expose him in his current state to the teacher, but the photo includes Jun, his sister, which causes Rumi to hesitate. She decides to flip the photo face down, but Seki doesn't just have 1 photo as he has several and Rumi ends up going through them and is distracted enough that she is caught off-guard when the teacher selects her to solve the problem on the blackboard.

Seki has an old tea-serving doll, that he has dressed up to be a cafe employee instead and to carry coffee instead of tea. The doll moves quickly and can manoeuvre around tightly packed tables with ease, although its speed does cause some of the coffee to fly out of the cup and hit Seki. He wipes his shirt clean and leaves the handkerchief in the path of the doll, causing it to trip over. Incensed by Seki's poor treatment of his doll, Rumi takes the doll away from him and refuses to give it back.

Seki is sick and therefore absent from school, much to the delight of Rumi who can now study in peace. Or she would, if she had remembered her Social Studies textbook, but luckily, Seki leaves his at school, so she borrows it instead. However, there's an issue with his textbook as the picture of Minamoto no Yoritomo (1147-199, first shogun in Japanese history to have de-facto rule of Japan) has an “X” next to it and the the flat staff he normally has is missing from the picture as well. Minamoto no Yoritomo is not the only historical figure to have their picture altered, as Takeda Shingen(1521-1573, famous samurai and military leader and was known as the "Tiger of Kai") is missing his fan and the reason is that Phantom Thief X (a character of Seki's own creation) has stolen both Minamoto no Yoritomo's staff and Takeda's fan. This ends up having an effect on history as without his fan, Takeda cannot block the blow from Uesugi Kenshin's (1530-1578, Takeda's chief military rival) sword and gains a facial injury in the process. Phantom Thief X isn't finished stealing yet as he steals: Amakusa Shiro's (1621-1638, Japanese Christian leader of the Shimabara Rebellion) ruff (white frilly collar which was fashionable in the 1600s) which allows him to fly, and Commodore Matthew Perry's (1794-1858, American Navy officer) fringed Epaulettes which means Phantom Thief X is the one that demands that Japan open its ports instead of Commodore Perry. Rumi is conflicted as one hand Phantom Thief X is evil by stealing other people's possessions, but because his actions have an effect on history is cool. However, Phantom Thief X meets his untimely end after being stabbed due to a case of mistaken identity. He had stolen the moustache of Itagaki Taisuke (1837-1919, Japanese politician who led the Freedom and People's Rights Movement and argued for parliamentary democracy in Japan) and thus suffers the same assassination attempt as Itagaki but unlike real life politician, Phantom Thief X dies. The lesson ends with no studying from Rumi, but this will be the only time today until Rumi realises that Phantom Thief X was reincarnated into this world as he has stolen the glasses of Taki Rentaro, whose portrait hangs in the Music classroom and she fails to focus on studying for the rest of the day.

Seki seems to be in pain, but it is of no worry as he is only massaging his hand. He is also massaging his feet on a bamboo foot massager and then moves onto a dry towel rub-down (with his clothes on of course). He then makes a herbal tea before soaking half of his hands in mineral salt bath. After a while, Seki's skin looks glossier and more healthier as well as being able to hit inside-the-park home-runs during P.E suggest that his new health routine has worked and has Rumi wanting to try it as well, however the other boys think he looks weird, so he immediately stops it.

Seki has got some stuffed animals out, but all are in need of repair, so he begins to repair one of them, Billy Bear, which is popular with people (Rumi has one at home). To help with the repair, he takes stuffing from one of the other stuffed animal, a stuffed cow, and uses it as replacement stuffing for Billy. He then moves onto another popular stuffed animal, Mitty-chan, and completely ignores Miss Cow (named so by Rumi), so Rumi takes it upon herself to fix Miss Cow instead. Despite being a girl (a justification for her decision), Rumi stitching makes it more horrifying than cute so Seki hands her some doll accessories that she can use to make Miss Cow look like a Mitty-chan. Rumi considers this, but in an effort to maintain Miss Cow's originality, she instead makes it a hat and scarf and this takes a lot of effort and concentration. When she wakes up, Seki is finally mending Miss Cow, but its hat and scarf have been given to Billy Bear and Mitty-chan respectively and he refuses to return those items of clothing back to Miss Cow, despite Rumi protestations, who now realises how hard it can be to become a surgeon.

Seki is playing with 'dolls' again, although these seem to be closer to cardboard cutouts of kids wearing the jersey of the national Japanese football team and he is dirtying them up. He has also modified his desk so the kids can move left and right in the same way the players of table foos-ball move. There is 1 goal on the table, but the kids don't kick the football into the goal as instead they kick against the wall and Maeda's back, dirtying both as the ball is also dirty. Rumi tries to stop the game by knocking the football with her rubber, but it is deflected and dirtied. Annoyed by her failure, Rumi tries to ignore Seki and believe it will sort itself out (if Maeda is concerned about the state of his shirt, then it is his fault for not noticing), but realises she hasn't told Seki off for not studying. The teacher is coming round to check on everyone's work, so Rumi waits until the last possible second to call him over, but Seki moves faster than their teacher, so he is not rumbled. Rumi changes attack and begins to fire rubber bands with sewing thread tied around them and Seki assumes she's after the ball, but instead she is after the kids. Rumi lassos the kids, but Seki has another trick up his sleeve as the jerseys are removable, so the kids can still play. Rumi gives up.

It's time for a cooking lesson and Seki, Rumi, and Goto are all in a group making potato dishes (no further elaboration). Rumi is happy that Seki playing around, although she knows he can work quicker and would like him to do so. She notices swift move and assumes that he is sneakily snacking on the ingredients. Move observation reveals that he is bending below the table and dropping the food into his bag, which makes Rumi conclude that he has some animal in the such as a cute cat or dog that he has recently rescued (Seki would not bring animal to school without some method of feeding it). She begins to help Seki by leaving a bit of food for him to take, but this assistance is spotted by Goto and whose thought process goes from spoiling partner with food to embezzling money. To nip this bad behaviour in the bud and prevent a future embezzlement crime, Goto blocks Rumi from grabbing ingredients by picking them up before she can. Rumi briefly comes to her senses but spots the milk that are supposed to use for the stew and the Seki is occupied away from their area, so decides to take the milk to the animal Seki has in his bag or, as it turns out, animals. Seki has a an ant colony in his bag (in a case, the ants aren't roaming free inside) and they, in Rumi's eyes, are not worthy of feeding and prevents Seki from handling any more ingredients and later apologises to Goto for her earlier behaviour.

Seki is being secretive, but Rumi can only see paper on his desk so is confused. Seki reveals what he is doing when he throws a low-flying paper aeroplane (high-flying would be easily spotted by a teacher), which lands at a runway on the other side of the classroom. To stop him, Rumi moves her bag to hang off the back of the chair, to block the path of the aeroplanes but Seki builds paper aeroplanes that can fly even lower. Rumi hangs her jacket off the back of her seat to close the space even more, Seki makes a plane out of sticky tape that can fly high without being spotted by the teacher (ignore that sticky tape isn't as rigid as paper and would fail to maintain its shape when hit by the air). Realising that she us about to enter an arms race with Seki, Rumi imposes a 500 yen (£2.39) fee to fly in her 'airspace' and luckily Seki can't afford such fee but unluckily, Seki tries to sell her some paper aeroplanes which is more annoying than Seki throwing paper aeroplanes.

Seki has laid out a tablecloth and tableware (glasses, fancy cutlery and crockery, even a napkin) as today, Seki is practising good table manners. Seki is currently practising how to eat soup with a spoon, with a specific interest on how to do it quietly and if you're wondering, to eat soup quietly, you bring the tip of the spoon to your lips first. Next is to how eat bread rolls when there is no bread plate (off the tablecloth is fine, assuming it is clean) although for Seki, the bread rolls are fake. Next Seki moves onto eating a steak with a knife and fork, but when trying to practice, Maeda gets up to answer a question and his chair clatters into Seki's desk and knocks the steak off the plate. Seki attempts to return the knocked steak to the plate, but realises the correct thing to do is to wrap the spilled food in a napkin and call for a waiter/waitress who will handle it from there whilst giving you a replacement napkin. The proper tables manners are not put into practice when Seki has his lunch later that day.

Seki is being secretive with what he's doing as he is sitting next to Uzawa (or Chaos Incarnate) and what he is doing is playing small carp streamers on a stool next go him, which are cone-shaped kites that are dressed up like carp. The carp need wind to move, but Seki has found a solution: attach the pole which the carp streamers hang from to a motor and get them to move that way. Seki adds more carp streamers, things to hang them off and motors to create a carp-themed theme park, which seems impressive, but Rumi believes that he is exploiting the carp. Seki is called to the front, leaving his theme park unguarded so Rumi takes the chance to steal all of the batteries powering the motors and if Seki asks about them, there is Uzawa as a scapegoat. When Seki returns and notices the batteries he first asks Uzawa, who denies having any batteries, but before he can interrogate Rumi but has worked out that she is guilty, Uzawa puts 2 and 2 together and wishes to play with Seki's battery-powered toys. Seki then quickly kicks the stool away into Rumi's knees, who accepts the pain as punishment for stealing.

Seki is late because his sister's (Jun) pre-school is temporarily closed and she was supposed to stay with an neighbour but she couldn't bear being separated from her brother, so she has come to school. She sits in between Seki and Rumi and watches while her brother smelt fishes with a small bowl of 'fish' placed on the shelf below the top of his desk and a hole in the desk top cut out to fish from. He doesn't involve Jun in any way, so she 'joins' in by making a fishing net made out of rubber bands and fishes a whole bunch of fish from the bowl. This means Seki's playtime would end very quickly, so he gives up and begins sulking much to Jun's disappointment. Seki then gets Go pieces out and draws a board on his desk and begins to practice various Go moves, whilst not including Jun, so she plays with the other pieces, making a line with them. Seki responds to this by making a line of his own Go pieces and the 2 begin to play Go by trying to occupy as much territory as possible. Rumi offers her desk to Jun and she moves over to take advantage, but exposes her chair to be claimed by Seki. Jun then begins to claim Rumi's lap and open palm, while Seki makes a move for Rumi's head by flipping pieces onto her head. They run out pieces and begin working out who claimed the most territory, but Jun falls asleep on Rumi's lap before a winner calculated, so Seki leaves his sister in Rumi's care and plays a new game with the Go pieces.

Seki is wearing down 3 rubbers: 1 green , 1 blue, and 1 white in order to squish the scraps together to make a miniature Earth which, naturally, is accurate in its appearance. However, to Seki, it is not finished as he drops it on the floor to get dirty which Rumi interprets as pollution that currently plagues our world. He rolls it around on the floor to get it even more dirty, but has to stop when handouts are handed out to him which causes him to lose sight of the rubber Earth. Rumi has spotted it, as it is by Maeda's feet and thinks about telling him but figures that it doesn't matter as he will inevitably make a new one anyway. However, their teacher starts talking about how environmental pollution is everyone's problem and cannot be ignored, which inspires Rumi to protect the 'Earth' by getting Maeda's foot to step on her own rubber than the Earth, with her explanation given to Maeda starting rumours about her protecting the Earth.

Seki is tinkering with a clock that is attached to a cylinder which looks like a bomb straight out of a movie, when he accidentally hits something and causes the timer to go off. Rumi is concerned with the damage it could cause if it goes off and implores him to disarm his bomb. He begins by cutting 1 wire, but that speeds the timer up so he moves onto the other wires: 1 red and 1 blue. He asks Rumi which one to cut but she refuses to answer, so he instead asks what shaved ice syrup does she prefer: Strawberry or Blue Hawaii. She, again, refuses to answer so he has to choose the wire himself. The compartment opens and nothing happens but Seki gets out a make-up kit and makes himself look like a zombie as what was inside the bomb was a zombie virus. He passes the make-up kit to Rumi who swats it away and out of the window.

The first bonus chapter continues on from 79th Period and features Rumi and Jun having lunch, with the latter having the face of Dad-bot on her lunchbox. Rumi then wonders if there is merchandise of the robot family and if so, there would be a Tv show that would inevitably show the robot family fighting (because we can't have a slice-of-life anime featuring robots, can we) and that would not do. However, the lunchbox is not robot family merchandise, but instead decorated with a sticker made by Seki.
The final bonus chapter is once again autobiographical and is the author's thought about the anime adaptation that aired just before the publication of the volume in Japan. He talks about what was his favourite episode (20th Period - Flip-book, which was 34th Period in the manga), how the voice actress for Rumi inspired the plot of 71st Period, and how he feels about having so many primary school students reading his manga.