Seki has come up with a cool new yo-yo trick and is trying to name it. He comes up with the name "Seki Big Bang", but it doesn't feel explosive enough to warrant such a name. He then goes for "Wild Seki Bird" instead, but that doesn't fit either, but overhears the teacher talk about the words "hide" and "seek" (the lesson is English), so names his new move "Seak King", although that name is A: the misspelling of the english word "seek", and B: a Pokémon (although that one is spelt with 1 'k'). However, the name is approved as the way the yo-yo appears and disappears from behind Seki's hands makes it look like an impish king playing hide and seek (Rumi's explanation, not mine).

Seki has a carpenter's plane and is using it to shave thin pieces of wood and when I say thin I mean translucent levels of thin. Its 'thickness' is 10 micrometres (0.001 millimetres), which for context, the diameter of a strand of human hair is about 80 micrometres. This is, however, to thick for Seki as he desires wood shavings that are 5 micrometres thick, so he continues to shave the block of wood and let the thick scraps float away onto Rumi's desk. Rumi tries to write on a 10 micrometre scrap, but it breaks, which makes sense when the standard thickness of paper is 50 micrometres, so she brushes them off her desk whilst being continually annoyed that Seki is causing a mess on her desk. Eventually, Seki shaves a scrap that looks to be that golden 5 micrometre (which is amazing, seeing as humans can at best see down to 100 micrometres from about 25cm away from the eye which Seki probably was to that scrap) but it floats onto Rumi's desk and in his haste to measure it, he hits Rumi's hand with his measuring device and she refuses to hand over the scrap (she slams a book down on top of it to deny Seki access to it) until he apologises for hitting her. He eventually apologises properly and Rumi lifts her book up, but the scrap has been ruined to shreds, which annoys Seki.

It is too hot inside the gym at school and everyone is suffering except Seki, as he is shivering after leaving the store room. Rumi goes to investigate but is scared when she hits a bar of soap when trying to find the light switch. The light switch is irrelevant as there is a torch by her feet to illuminate her surroundings, which she regrets as she freaked put by a body science model in a vaulting box. Realising that Seki has built a haunted house out of things found at the school, Rumi is a bit more relaxed, although a skeleton taking an eye exam and piano music played from hidden speakers do catch her off-guard. She eventually finds the exit, but cheerleading pom-poms cover the door so she has to find the handle, but opening the door knocks the pom-poms down and reveal a poster of Beethoven (terrifying). In a blind panic, Rumi throws the poms-poms away and they begin to form the own creature which terrifies her further as she leaves. The pom-pom creature is actually an unfortunate Seki.

Seki has found something to on the way to school to play with the Robot family, which he plays with during morning home-room. The 'thing' is a robot dog that dwarfs the other robots and kid-bot wants to keep him, but his parents aren't fully convinced despite kid-bot demonstrating that robot-dog can perform the the 'shake hands' trick. During the first lesson, Seki plays with something that isn't the robot family, which frustrates Rumi but deduces that the robot family wasn't the main idea for Seki, so he will only play with them during breaks, so is extra vigilant for the rest of the day, especially during the break periods (which is after every lesson). Seki doesn't play with them during 1st and 2nd breaks, but does during the 3rd but robot-dog has weak front right ankle which comes up when gives away whilst kid-bot is riding him. Kid-bot has a broken shoulder and parents are now more distrustful of robot-dog, but it is now lesson time, so the story is on hold. The robot family are not brought out for the rest of the day, but after noticing Seki putting kid-bot in his shoe locker, Rumi discovers that both kid-bot and robot-dog are in the locker as both have run away from home. Rumi decides to take the 2 home, but hesitates and is interrupted by her friend and thus drops robot-dog and kicks it by accident.
The next day, Seki and the robot family discovers the robot-dog is in a mangled form but Seki is able to correct it as it turns out, robot-dog was in mid-transformation to a humanoid robot. Despite no longer a suitable pet for the robot family, Seki keeps robot-dog within his family as Jun is seen outside with it.

Seki is sorting some acorns and is putting some fake acorn caps on some and packaging those into a fancy box to marketed as premium acorns despite coming from a value bag of acorns. Some of the acorns drop onto the floor and Rumi picks them up to return, but Seki isn't interested them as they aren't 'ultimate' acorns. Rumi decides to play Seki at his own game and ties the acorns she picked up in tissue and markets her as all-natural acorns that are cheaper then Seki's, but there less of them (Rumi's has only 3 acorns to Seki's 6, so Seki's ultimate acorns are cheaper per acorn). Seki then ups the quality of his acorns as markets them as acorns from 1,000 year old tree that have ultimate longevity, so Rumi counters by writing a health talisman on her acorn packaging.

Seki has made a treasure map and is trying to work what treasure to put in the chest. He puts some of his old toy cars and motorbikes, but he doesn't feel like that is a good treasure, so he instead decides on another treasure map, but that just kicking the can down the road and the boys will use this treasure map aren't going to be pleased about doing another one. He looks through more of unwanted things and finds a small glass bottle holding some "mystery seeds", which are pits of dried plums that form part of Seki's lunch and these are the perfect treasure as the young treasure seekers will forget about the plant that would grow from these "mystery seeds". Rumi thinks otherwise and disposes of the pits during a break between lessons that day.

Seki has a vanity mirror and is trying on adjusting a fake white beard as well as a red Santa hat. However, this is not accurate enough for Seki as he applies make-up to make him look even older. Seki looks completely unrecognisable, but doesn't stay that way for long as he is selected to read something to the class and must hastily return to normal. He then moves on to dress up and be like a reindeer, which means attempting to look cross-eyed and this is of no help to Rumi who is reading whilst he is doing this and must excuse herself from the classroom.

Seki isn't finished with the vanity mirror used last chapter as he has stuck a dartboard sticker on it, so it reflects a dartboard for him to throw things at. He points it at Maeda's back and throws burrs (specifically cockleburs) at the dartboard. This may be fun for Seki, but it is irritating Maeda, who removes them from his back and tosses them back at Seki without even looking (behold the powers of an ace pitcher - Shohei Ohtani watch out!). Despite knowing the consequences of such actions, Seki continues to push hi luck and continue to throw his burrs at Meade's back but his first burr is intercepted by Maeda who knocks it back with the precision of an ace batter (Shohei should really watch out then) and it strikes Seki in the chest, right where his heart is. Seki learns not throw burrs at Maeda's back and chooses his jacket on the back of his chair instead. Maeda throws them at Seki's back later so it spells "dummy" in Japanese during the next break period.

Seki has a film camera and a cardboard model of a generic city skyline as he is filming a monster movie, which sees Sekidon appear in Tokyo. Sekidon is not the only monster in town (literally) as another appears from under the ground and asks if Sekidon has been waiting long, which he hasn't, with this all being communicated via speech bubbles. The 2 monsters are on a date (yes, this a monster romance movie) and the lady monster (she is never given a name) is wearing a hair accessory that Sekidon appreciates, although Rumi can't see it literally (you're such a bad boyfriend Rumi). They set off for to see the movies, but Sekidon walks faster than his date, so she has to jump on his tail and knock him over (and into buildings) to get him to stop so they walk at the same pace. Sekidon turns around to begin walking again, but his tail knocks the lady monster causing untold destruction and devastation. Seki show this destruction by: himself acting scared, rotating the camera and filming Maeda so it looks like he is laying down after being hit by flying debris, and also gets some reaction shots Rumi in a similar vain to his acting job earlier. Sekidon get down on the ground looks like he is bowing but is instead trying to pick up and protect his date's hair accessory which ad been knocked off when he literally knocked her off her feet. The 2 monsters make up and the film ends with them looking at a sunset over a flattened city.
Next lesson, Seki decides to film the sequel, but Rumi refuses to return to the series.

Seki is playing with a smartphone or more specifically, a sticker of one and places other stickers to represent different apps or new phone case. One of these apps is a mail app with which Seki receives a threatening email, which seems unbelievable until you spot a bullet hole in the window next to you and the textbook you were holding. Seki's assassin fires another 'bullet which hit Seki right in he chest, but doesn't kill him as his smartphone catches the bullet (shown as a sticker) preventing any injury. The cleanup from this play session is little more difficult as it is hard to get stickers off of windows.

Seki is whittling some pencils down so that they can be interlocked with each other and form a double-sided pencil which seems revolutionary until you realise such pencils already exist. He then pulls a blueprint for another invention that will disrupt the pencil industry: Don't you hate it when pencils become too dull to use? Well at Seki Pencil Company (unofficially named and not real) we have the writing implement for you! for $10 a month we will rent you our new 'Combi-Pencil' that when pencil lead becomes too dull you can pull it off and you will have a brand new sharp lead ready to use! Ignore that such a device already exists on the market, known as a pop-a-point pencil. But Seki Pencil Company is not 1 trick pony either,as we have another design that solves a very common problem: sharpening pencils takes too long. We have invented a pencil that you can push the lead down the pencil so you only need to whittle the lead to resharpen it - very revolutionary and disruptive to the stale pencil market! We have named it the 'Speedy Sharp Pencil' and it will be available for $15 a month! Ignore the well established mechanical pencil which does the same thing only better as it requires no sharpening of the pencil. Our founder and Chief Product Officer, Toshinari Seki, will now demonstrate the 'Speedy Sharp Pencil' in action so you can see it effortless write like any other pencil until the lead snaps fairly on into usage. Well, teething troubles are to be expected with outside-the-pencil-box thinking, but I am sure this can be fixed by Seki and his lost motivation for anything, so we are filing for bankruptcy and closing down as Big Pencil couldn't handle the truth that their complacency was holding the industry back and needed to maintain its monopoly on the market.

Someone from Rumi's class has left their art textbook in the art classroom and Rumi has been tasked in finding its owner. This is solved pretty quickly as opening the textbook reveals the letter 'X' - Phantom Thief X has struck another textbook, so this textbook must belong to Seki. Phantom Thief X is back up to his old tricks again as he has stolen the flute from Édouard Manet's (1832-1883) "The Fifer". Phantom Thief X isn't just interested in musical instruments or physical objects as he steals Mona Lisa's (Leonardo da Vinci's famous painting) smile before stealing the Rubin Vase, a bi-stable optical illusion created by Edgar Rubin (1886-1951), so all you get is a black square and no specifically shaped vase (the manga uses the word urn, which is incorrect regardless if the Japanese original used that word). Also Phantom Thief X is seen without his jacket as he has handed it to Venus in the "Birth of Venus" painting by Sandro Botticelli (1445-1510), to cover herself up. Phantom Thief has moved west and forward in time as he next found in Pablo Picasso's (1881-1973) "Guernica", although the Cubism gods don't take kindly to his free-spiritedness and have trapped him within the painting. Luckily for Phantom Thief X, Rumi comes to his rescue and draws him escaping the cubist ruins of Guernica, but Seki erases Rumi's efforts.

Seki is juggling 3 sheep beanies and can do it quite quickly, but he does drop one of them. He eventually puts them down and puts a wolf hand puppet on, so Rumi instinctively takes the sheep away from the big bad wolf (Seki). However, these sheep a hot to the touch as they are actually hand warmers nad Seki's earlier juggling has activated the chemicals inside. She tries to put them down on her desk, but that puts them within the reach of the big bad wolf so she must continue juggling them despite such actions make them even warmer. Seki lunges in closer, so Rumi pushes the sheep hand warmers in his face, which gets him to cool his attack and she can safely hide the hand warmers in her bag at the cost of a heated lunch.

The first bonus chapter Goto and Rumi discussing Seki and Rumi's idea of getting evidence of catching Seki in the act, which Goto obviously misinterprets and shoots said idea.
The final bonus chapter is autobiographical as Takuma Morishige answers the question: Did you goof off in class as student? The answer is no, he saved such antics for break times and was never as elaborate as Seki although he did get into obsidian honing, which seemed to be school-wide trend that concerned the teachers enough to ban sharpening stones at school.