The story of HELL GIRL:
Most episodes are self-contained short stories in which the series narrates the suffering of a different individual caused by one or more antagonists. A website known as Jigoku Tsūshin (Hell Correspondence) may only be accessed at midnight by one who harbors a desire for revenge against their tormentor. Should someone submit the name of someone against whom they have a grudge, the Jigoku Shōujo (Hell Girl) will offer them a straw doll with a red string wound about its neck. If the string is pulled, she will ferry the target of the revenge straightaway to Hell, however the client agrees to a pact where those who request her intervention will also be sent to hell at the end of their natural lives.
During each story, the protagonists' dramas are explained in detail from the start of their grudges, through the escalation of their torment until it becomes unbearable and they resort to accessing the Hell Correspondence website. The contents of the site are only a text: "あなたの怨み、晴らします。 (We will take revenge on your behalf.)", a text box where the grudge's object must be written, and a "送信 (Send)" button. Some time after the post, they are visited by Enma Ai, a young red-eyed girl wearing a traditional sailor school uniform (usually the dark version used only in winter months). She hands them a straw doll, which is actually one of her assistants, with a red string tied to its neck. In the first season, the doll is always black, because it is always the same assistant, but in the second season, the doll may also be red or dark blue, depending on which assistant it is. She then tells them that if they want their vengeance to be delivered, they must remove the string from the doll, and their enemies will be immediately taken to Hell. A black crest-shaped mark will appear on the protagonists' chests, which serves as a constant reminder that once their lives come to an end, they must give compensation for Ai's service by having their own souls also sent to Hell.
Ai is aided by her three assistants: Ichimoku Ren, a young man in casual clothes who takes the form of the blue straw doll; Hone-Onna, a woman wearing a kimono with the obi (sash) tied in front which signifies she is a prostitute and takes the form of the red straw doll; and Wanyūdō, an old man wearing a hat and, at times, a red scarf, who takes the form of the black straw doll. When not actively assisting Ai, the three remain in their straw doll forms. The trio help Ai investigate the true nature of their clients, and also present the victims with the sins they have been accused of before Ai appears to ferry them to hell.
In episode 8 of the first season, two recurring characters are introduced: A journalist named Shibata Hajime - a former scandal-hunter/blackmailer who turned to investigating the stories involving the Jigoku Shōjo; and his daughter Tsugumi. After an encounter with Enma Ai, Tsugumi starts to have visions of what Ai sees and thus the two become more and more involved in Ai's matters. Hajime doesn't agree with Ai's methods to deliver vengeance for her clients, and tries to stop those who contacted Jigoku Shōjo from using her service with Tsugumi's help.
In season two, the major plot line revolves around Takuma Kurebayashi, a boy who is blamed by his townsfolk for causing disappearances around the town, which was actually caused by the townsfolk who used Jigoku Tsūshin.