Takagi loses his notebook after breaking up a fight between a manager and his singer. He searches for his book with the help of the Detective Boys. They then discover that the singer plans to murder his manager. The detective boys manage to bring Takagi to his house and save him before he was killed. With the manager was Takagi's notebook.
Hanzaki kept checking the clock rather than eat at the all-you-could-eat as he energetically claimed he would.
Hanzaki bought a block of ice and a slab of concrete, which was used to create a mechanism which would eventually drop the concrete upon the manager Sugiyama's head. This was found in his apartment bathroom, along with Sugiyama.
Hanzaki purposely asked Takagi for the time, even though there was a clock right in front of him in the park.
Motive
Hanzaki was blaming the manager for his lack of success.
Conclusion
Hanzaki was in fact attempting to kill his manager, and was planning on using Takagi to help build his alibi.
The murder was to take place in Hanzaki's apartment's bathroom.
Ending
The Detective Boys and Takagi arrive just in time to save Sugiyama. As it turns out, Sugiyama was the one in possession on Takagi's notebook, whereas Hanzaki just observed Sugiyama doing so. Hanzaki is promptly arrested after being confronted by his manager and Takagi.
Unfortunately, Takagi gets in trouble anyway because he claimed he was sick and took a day off to find his notebook.