@sejintenejThe most important things to understand when contemplating these issues is that plagiarism is an academic standard, copyright is a legal standard.
For the most part, no one who isn't in an academic community cares about plagiarism. It simply doesn't matter. Oh, your community may think badly of you and it could cause bad press, but it doesn't matter in any real-world legal or economic sense except in very rare situations.
By rare situations I'm talking about things like someone plagiarising in their thesis to get a degree, using that degree to get a job, then being found out. The plagiarism leads to a court case where they retroactively lose their degree then get fired and sued. It's still all linked to academia, though; the employer isn't firing them for plagiarism, but for fraud.
Even where plagiarism does apply, it refers to using someone else's ideas or text without permission or credit. If you quote a short passage of an existing work and use is to make your own point, it's enough to reference the original author through a citation. If using a large portion of an original work it is sometimes necessary to get permission or give the originator a co-writing credit, even if they did no new work.
So unless you're a student, what matters is copyright, and copyright is exactly that: the legal right to control how and if copies are made. Specifically, copyright applies to "any creative work fixed in a tangible medium" (phrasing will vary by country).
Importantly, ideas cannot be copyrighted. What is copyrighted is the specific expression of an idea. Also, individual characters, recognisable names, elements of world design, etc. But not the underlying story, concept, plot, or characters arcs. You also can't copyright anything factual.
So what all this means for your question is that for aroslav to write a story based on someone else's idea is neither plagiarism nor copyright.
For plagiarism, if aroslav mentions the original author of the story then that's all that is required.
For copyright, the few lines describing the original story might be a copyright violation if quoted exactly, but not if described generally, and the underlying idea cannot be copyrighted. All of the original work done by aroslav, ie. the actual story, is also a new copyrightable work.
So it is theoretically possible that there could be a copyright violation involved, but it's not that part you think it is and it definitely isn't plagiarism.