"Jigoku Tsushin," a website that only accessible exactly at midnight. If you post a hopeless grudge there, Hell Girl will appear and drag your hated foe into Hell. The rumor sounded like an urban myth, yet it spread among the young people and turned out to be true.

The girl's name is Enma Ai.

When she sympathizes with a client's grudge, she deems their foe a target, and sends the target to Hell as Hell Girl. However, the legend does not speak of the contract with the Hell Girl. Once you have your retribution, you will have to pay the price for it as well. Curses come home to roost. If you enter into the covenant, your soul will also go to Hell. That's only after you die, though.

And then one day, Michiru, a mysterious girl appears in front of Enma Ai. Ai talks to Michiru, who doesn't remember who she is or why she is here. Who the hell is she? Her secret past will be revealed!

Source: crunchyroll

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Prequel: Jigoku Shoujo Mitsuganae