Life wasn't easy under the occupation of the Erionians, a highly technological advanced human civilization from a distant exoplanet orbiting a quasi-star. About roughly ten years ago, the Erionians launch a massive scale strike against the nations, and it concluded by a decisive victory for the Erionians.
Despite their oppressive rule, the Erionians however always succeeded to preserve a high image on themselves to prevent local resistance. They often recruited the locals into their ranks and gave their full support to every groups and organisations who choose to allied themselves with the Erionians.
Inside a large hall somewhere in Research Facility No. 13, the 15-year-old boy Otto Kurokawa was helping his grandfather, Konstantin to build an inter-dimensional portal machine. It's unknown why their Erionian overseers ordered the workers to build a lot of these portal machines since they never give any reason...
Unless if there's something the overseers refused to tell.
"Finally, this portal machine is almost completed" Konstantin wiped his forehead in relief.
"Well, i guess you can take some rest in the weekend, right?" Otto smiled.
"Yeah" Konstantin nodded. "Perhaps we can go fishing or camping after this"
"Well, i'm just worried if my fishing rod got broken again" Otto scratched his head.
"Don't worry, it won't happening again" Konstantin chuckled.
As Otto was checking his smartphone, he suddenly realized something.
Today was the birthday of Konstantin's former classmate, Sakonji Urokodaki.
"Grandpa, can we go to Urokodaki-san's house later? Today is his birthday!" Otto reminded.
"Oh boy, i'm almost forgot about it" Konstantin sighed. "Let's clean the laboratory first before we go"
...
"Thanks for coming, Kurokawa-san!"
Otto and Konstantin were greeted by Sakonji's grandson, Hiroshi. They just arrived recently at the Urokodaki residence.
"I'm sorry that we almost forgot about your grandpa's birthday, Hiroshi-kun" Konstantin apologized.
"It's okay, please come inside!" Hiroshi invited.
Meanwhile, as the Kurokawas entered the house, Inosuke already ate all of tempuras on the table.
"Inosuke, don't finish all of the tempuras by yourself!" Aoi scolded.
"But i'm hungry!" Inosuke replied.
"At least spare some food for us!" Zenitsu yelled.
"NO!" Inosuke refused.
And not far from them, Otto soon noticed a certain male voice called his name.
"Otto!"
When Otto turned his face, he noticed Tanjirou waved his hand towards him. Tanjirou's younger siblings, Nezuko and Takeo stood next to the hanafuda boy.
"Tanjirou" Otto quickly walked towards them.
"Otto! I'm glad that you're coming!" Tanjirou smiled.
"Sorry, my grandpa and i are very busy today" Otto apologized.
"It's alright" Tanjirou said.
"You guys don't have any mission today?" Otto asked.
"No" the Kamado siblings shooked their heads in unison.
"Finally we could find a day to rest ourselves" Nezuko said.
"Kurokawa-shonen!"
Out of sudden, Otto noticed someone's hand was touching his shoulder. When he turned behind, it was Kyoujurou.
"I haven't see you for a long time! Where have you been?" Kyoujurou asked the younger boy.
"Well, my grandpa is much busy this month, and i need to help grandpa in his job" Otto answered.
"That must be tough task for you, right?" Kyoujurou asked.
"Yeah" Otto nodded.
Both of them then looked towards Konstantin who conversated with Sakonji and Giyuu.
...
The digital clock inside Konstantin's car was showing 10:15 PM. Konstantin was driving the car with Otto sat next to him as they were on their way home from Sakonji's house.
When Otto glanced at his grandfather, his face was looking depressed sometimes. As a caring grandson, he felt a great sense of worry towards his grandfather.
"Grandpa?"
Konstantin noticed Otto was calling him.
"Yes, grandson?"
"It seems that you're troubled by something" Otto commented.
Konstantin took a deep sigh before finally spoke.
"Don't worry about me, grandson. I'm fine" Konstantin assured.
"Are you sure, Grandpa?" Otto asked worrily.
"Oh, one of my colleagues got sick, and i'm concerned about him" Konstantin replied.
Otto however felt skeptical about his grandfather. He knew, the elderly Kurokawa was hiding something from him.
"Oh, if you-"
Out of sudden, something unexpected just happened.
A mysterious person wearing a mask just crossed in front of Konstantin's car, carried a sword.
"Who... who's that!?" Konstantin was shocked.
If Otto looked carefully, the pattern of that person's sweater was very similar to Tanjirou's necktie. Black and green checkered one. And that person also wears earrings, just like Tanjirou.
Wait a minute. Tanjirou never wore a mask during his mission, and there are no specific uniforms for the members of the Infester Slayer Corps.
So the question is, who's that person?
To be continued...
Fun Fact #1:
1. The original first name of Konstantin is Hideki.
2. Otto and Konstantin are Catholics.