Reimu Hakurei, the Hakurei shrine maiden, is doing all she can to resolve the Lost Word Incident. A red seal crystal sent her to another parallel world. What new adventures await her there?

The Scarlet Devil Tower was made by Patchouli to automatically construct and grow itself using magic. The tower starts with Flandre's room in the basement, then the Great Library and the Forest Zone, but an elevating machine is needed to climb any higher. Reimu and the others used Sakuya's ability to reach the gate that Meiling was guarding from the inside.

The lone girl found a goal that was grander than becoming a witch... It was to become friends with this strange girl, who was strong and could do anything she set her mind to. The lone girl wished with all her heart that she could become worthy enough to be her friend. She wracked her brain trying to figure out what it was that would make her "worthy"...

The Lost Word Incident turned the Marisa of the Scarlet Devil Tower world into the Witch of Scarlet Dreams. She became a powerful and fearsome witch who continuously spouted blazing heat. The Scarlet Devil Mansion had to be rebuilt into the Scarlet Devil Tower to distance the Witch of Scarlet Dreams from the earth.

"Tsk, you dodged everything," complained the Witch of Scarlet Dreams as she watched Reimu easily navigate through her curtain of blazing hot bullets. "Your bullets are much too hot for me," replied Reimu with a chuckle while making a fanning motion with her hand. Bright smiles lit the faces of both combatants. Even in a parallel world, Reimu and Marisa are rivals―and friends.