This article is about an episode of the Detective Conan anime that has not yet aired. As such, it is a reflection of what is currently known about the episode. More information will be added after the episode aired.
One morning, Nakatsu and Kuchiki were both found by construction workers inside a patch of hardening concrete. Forensics found that Nakatsu was suffocated by having his breathing cut off, but no murder weapon was found. Upon being questioned, Kuchiki has claimed that Nakatsu was actually killed by an alien hovering above them; some credibility for this tale is lent by the fact that the only footprints found in the concrete were Nakatsu's and Kuchiki's.
Yusuke Kuchiki murdered Nakatsu because the latter had derided his latest effort in taking an actual UFO picture; the trigger for the murder was that Nakatsu suggested that he could submit even a picture of a fake UFO. Kuchiki used a solar balloon - a black, cylindrical plastic bag which floats if the air inside is warmed sufficiently - as the murder weapon, cutting off Nakatsu's breathing and forcing him into the concrete patch. Following Nakatsu's demise, Kuchiki let the balloon float away into the nearby park, where it eventually ended up in one of its trees, and then dropped himself onto his own deepened footprints (which would have betrayed his perpetration), erasing them. He merely concocted the "alien murderer" story in order to ward suspicion off himself.
However, Kuchiki committed a number of mistakes which eventually lead to his arrest:
In order to faciliate his plan, he practiced the disposal of the murder weapon, leading to Hina's purported UFO sighting. The black practice balloon and several book about physics, in which the solar balloon effect was described, were found by Conan in Kuchiki's cupboard during the house search.
The fact that Kuchiki had pulled his hood over his head despite the warm summer season, in order to prvent his hair from getting stuck in the hardening concrete.
That he did not post his alleged "alien encounter" in the Internet, despite him being such an UFO fanatic, because he did not want anyone to find the "UFO" somewhere in the park.
The fact that, following the attempt at Natsuko Shibuya's life, security cameras were installed in the park where the solar balloon must have floated, rendering Kuchiki unable to retrieve it without being seen. The traces of Nakatsu's saliva and Kuchiki's fingerprints on the plastic would provide the crucial evidence of his murder act.
Season 25 (2020)