Anon is helping his brother's summer project. His brother want to make something with yukkuri, something that move and catchy.
He told his brother to look for cardboard box and some styrofoam. Then, he directed his brother to make the thing (so it's not too perfect that the teacher know someone else do it).
They then go to look for yukkuri in the park. Strays here usually hide from human, but looking for them is as easy as calling "Take it easy!", although only his little brother can yell that loud without any shame.
They found a Marisa who quickly told them she's not causing any trouble to Mister Human. They understand and asked if Marisa is the strongest. without thinking, Marisa answered yes, but then realized what she said and say "just among yukkuri", implying she know she's weaker than human.
The human then give Marisa a present to make her stronger. It's the cardboard box, layered with aluminium foil and plastic wrap. There's hole for Marisa to see out and her hat secured above the box. Underneath it are wheels from Suii and also a plastic sword-shaped ice cream stick in front.
Marisa loves it. The Suii is shiny, looks strong and waterproof.
They then go to the riverside. Unlike the park that has more food, the competition here is more intense and thus more scum.
A Reimu and KoReimu pair demanded food, so Marisa told them they shouldn't do that to human. Seeing them as scum, Marisa punished them with a ram after the human told him to.
Normally, ramming with the plastic knife at that speed would break Marisa's teeth and pierce her throat from the impact, but thanks to the cardboard box that absorbed the shock, Marisa is safe.
The scum pair runs up on stone. First, they need to recover from the uneasy feeling by looking down on the uneasy Marisa tank. They pour pee pee but Marisa suddenly jumps next to them, something they thought is impossible.
The mother is killed while KoMarisa escape to the water with the floating skill she learned from her father.
Unfortunately, the tank is waterproof and the bottom is layered with styrofoam to enable it to float. Marisa pushed the KoMarisa down to water and returned.
The human then told Marisa, she's now strong with the equipment, but never forget that human gave her that power and human can destroy it. Marisa should never thought of fighting human even in case of an extermination.
For a month, Marisa live with the family until Anon's brother presented his project to the school. Marisa is then released to go home to his clan where he's greeted like a hero.
A Youmu came with bad news that many strays are coming to their place. There has been an extermination. The survivors don't want to live in place filled with death stench so they escape to the park for refugee. The park's yukkuri know to not cause problem with human, so they're excluded from the extermination.
Their leader Patchouli is worried. If scum came in and caused problem, it could spell doom for them too, damaging the good relationship they have built.
Marisa suggested she will punish the scum.
Patchouli believed there's also good yukkuri among them, so she suggested to have interview first and Marisa can punish the scum there.
The scums are easily identified in the interview because they blame human for the extermination instead of acknowledging their own mistakes.
This way, the park clan managed to defend their life.
Unfortunately, a week later tank Marisa starved to death. The design of the tank has a weak point, which is, yukkuri can't open the tank. Unable to get out means Marisa can't eat, thus she died from hunger.