Nami Matsushima is used as a spy by her first real boyfriend, a police detective named Sugimi, to investigate a drug smuggling ring. However, her role is discovered and she is raped by several drug dealers. It emerges that Sugimi was simply using Matsushima as a pretext to obtain a bribe from the yakuza. Seeking revenge, Matsushima makes a failed attempt to stab Sugimi on the steps of the Tokyo Metropolitan Police headquarters. She is sentenced to do hard time in a women's prison, where she is given the number 701.
The prison is run by sadistic and lecherous male guards. The prisoners are forced to walk up and down a stair-like contraption naked with male guards watching from below. While incarcerated, Matsushima meets inmates like Yuki Kida, who was convicted for fraud and theft; Otsuka, jailed for burglary and extortion; and Katagiri, who was imprisoned for arson and illegally disposing of a body. Outside the prison, Sugimi and the yakuza orchestrate a plan in which Matsushima will succumb to an "accidental" death in prison.
The conspirators enlist the help of Katagiri and quickly set their plan in motion. Matsushima is attacked in the shower but defends herself, wounding the attacker. She is punished by being held bound by ropes in solitary confinement. A group of trustees, including Katagiri, tortures her; one pours hot soup on her. Matsushima is able to trip the trustee and make her spill the vat of hot soup over herself, causing horrible burns. Matsushima is forced to dig dirt holes for two consecutive days and nights. She kills a woman who attempts to attack her during this digging by tripping her and breaking her neck. In response, Matsushima is hung and tied from the ceiling while being beaten by her fellow prisoners.
After a prison riot, Matsushima escapes and kills Sugimi and all of the yakuza with a dagger. The film ends with Matsushima walking alone back in prison.
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