Beware of spoilers below if you haven’t seen the series yet!
Understanding the story
The interceptors and the decoy
It all starts with a detective in the Special Investigations Unit (SIU) squad of the Nihama police discovering that the detectives have had interceptors secretly put in them. Interceptors are machines put in the body that record everything a person sees and can transmit this information.
Superintendant Daido the chief of police did this with the help of a select few like the chief of Nihama police Nibu to secretly monitor the force. Specifically, it was to monitor the SIU and their findings concerning the ongoing laughing man investigation.
The detective gets killed when he discovers this (remember the flashing in his eyes that led to his car crashing) but with some luck he manages to give some photos as evidence to Togusa that used to work with him.
With these photos and the help of an SIU detective that talks to Togusa about interceptors, Togusa figures it out and section9 starts looking into the whole affair.