The Shirakaba-Sansō incident (しらかば山荘事件) occurred in 1970 and began with a radical university student group robbing a hunting store and stealing guns and ammunition. The group of students escaped and hid in a small private villa that functioned as a recreational facility among the mountains of Nagano-ken, taking the facility manager's wife hostage in the process.

After the police were alerted and arrived at the scene they were subjected to gunfire. Multiple attempts were made by the police to end the standoff, including hosing down the structure and using tear gas. Police were forbidden from using firearms. Attempts by the students' families to persuade them into surrendering were also ineffective. During the incident, Ōishi Kuraudo helped facilitate discussion between the local police and the Metropolitan Police.

Yamaoki Kaoru, serving on the Metropolitan Police force, eventually ended up taking command of the situation after the previous riot police commanding officer was fatally wounded by gunfire, starting what was to be the final attempt to end the standoff on a February morning at 10AM. The assault began with him commanding Minai Yūsuke to strike the villa with a wrecking ball, opening a hole between the second and third floors, and destroying the staircase between them.

Riot police threw tear gas into the building and charged in, apprehending 2 students from the first floor and rescuing the manager's wife on the second floor. A ladder was used from the outside to reach the third floor. After reaching the third floor and locating the room the remaining students were barricaded inside of, Maeno Haruyoshi was shot in the right kneecap and incapacitated by one of the students, Takiguchi Kazunori, who was patrolling the third floor. Before Takiguchi could follow up with a second shot, he was shot and killed by Minai Yūsuke.

In total, the standoff lasted 10 days, during which 3 police and 1 student were killed while the rest of the students were apprehended. Minai and Maeno were both summoned to the Metropolitan Police headquarters afterwards for questioning concerning Minai's fatal shooting of Takiguchi. Yamaoki then actively worked to protect Minai from repercussion and moved him to a different position directly beneath him, where he presumably started work investigating the smuggling of Plasil into Japan.