In the MMORPG Cross Reverie, the reclusive gamer Takuma Sakamoto is known for his unstoppable Demon Lord character, Diablo. His life of solitude is upended when he is suddenly transported into the game world, finding himself inhabiting Diablo's powerful body. He quickly discovers he was summoned by two young women: Shera, a bubbly elf, and Rem, a guarded pantherian. Their plan was to use an Enslavement Ritual to bind him as their servant, but Takuma’s unique Magic Reflection ability backfires, placing magical collars on the girls instead, making them his slaves.

With crippling social anxiety and no way to return home, Takuma decides to fully embody his in-game persona—a haughty and all-powerful Demon Lord—to mask his insecurities. He sets off on a journey with Shera and Rem to find a way to remove their collars. This quest draws them into the wider conflicts of this fantasy world, which is home to various races like elves and pantherians, as well as powerful beings known as the Fallen.

The narrative is structured around several key arcs. An early major storyline involves the invasion of the city of Faltra by the Fallen army led by Edelgard. Diablo's immense power turns the tide of battle, leading to an encounter with Krebskulm, the child-like Demon Lord of the Soul, whom Rem has a mysterious connection to. Another significant arc takes the party to the elven kingdom of Greenwood, where they confront Shera's cruel brother Keera, who seeks to force her into a political marriage. Later, the story delves into the corrupt inner workings of the Church of the Celestials when the group escorts a young priestess, Lumachina, exposing its dark secrets and battling paladins who abuse their authority.

The upcoming season, subtitled ULT, continues this story. The cast returns, including Masaaki Mizunaka as the voice of Diablo, with Yu Serizawa as Shera and Azumi Waki as Rem.