Sasaki Moritsuna has captured Koshiro, the young son of his brother Takatsuna, but worries that love for his son will tempt Takatsuna to go against the warrior code and betray his lord.
Moritsuna asks his mother to force Koshiro to kill himself, so that he will not be a temptation and threat to his father.
When Koshiro sees what is supposed to be his father's head, he stabs himself. But the head is not his father, it is the head of a substitute and Koshiro's death is part of an intricate strategy to manipulate Moritsuna into declaring the head genuine.