Yuko Ahso was once an ordinary high school girl. Or least her life used to be ordinary. Her friend Reiko went out on a date with a Dark King named Rogles, and has vanished. Immediately, monsters attacked her. She thought she would die for sure, but a mystic sword named Valis appeared before her. Using the sword, Yuko defended herself against the monsters, and was whisked to a strange land, where she was told that she was the chosen Valis Warrior and must defeat Rogles, or the human world and the dream world would both be plunged into chaos.

Valis: The Fantasm Soldier is a side-scrolling platformer. Yuko can jump and attack enemies with her Valis sword, which can be upgraded and used as a ranged weapon by collecting power-ups found in the stages. She can also find and use items that grant her powerful all-screen attacks or temporary invincibility. Each stage culminates with a boss battle, after which Yuko's hit points and attributes increase.