This episode sees the Detective Boys (minus Conan) stumbling onto a murder scene. It turns out Conan is at a hot spring with Kogoro, so Mitsuhiko is relaying information about the case to Conan over the phone.
The point of this episode seemed to be that it was trying to show that the other Detective Boys have the potential to be detectives in their own right. Obviously, they don’t piece everything together themselves. It’s ultimately Conan who figures it out, thanks to information and photos that he is able to acquire.
Overall, this episode felt like a regular murder mystery, except for the fact that Conan wasn’t physically on the scene for most of the episode. In the end, Mitsuhiko is the one who breaks down the information in order to reveal the one truth behind the mystery, but it’s still something that Conan deduced.
I think the writer had good intentions going into this filler episode, but I don’t think it was quite as strong as it could have been. In the end, it turned into an episode that just had Conan not physically present but still was the one who figured it out. It would have been kind of cool to have Ai be the one to solve the case instead of Conan. That, to me, would have really made this “a day without Conan.” But I guess the writer (or writers) felt that they couldn’t deviate too much from the regular Case Closed formula.
From the preview, it looks like the next episode will also be a filler story. Hopefully before long, the series will finally return to canon material from the manga.