Believer! is an otome visual novel described as a "battle fantasy" where you play as a young sorceress protecting the world from supernatural threats. When a dimensional door opens and mysterious beings begin attacking, you discover your true role as a miko with special powers. Five guardian characters have trained their entire lives to protect you from these forces. Their fates intertwine with yours as you uncover the truth behind your mysterious abilities and the connection to the parallel world. The game features voice acting with prominent seiyuu cast and character routes averaging 2-3 hours each.

"Once in a thousand years, the door to a parallel world opens, bringing strange beings into our world. However, when this time comes and the world falls into chaos, the legendary miko* and her guardians will be our saviours--"

Following the prophecy that has been passed down for several thousand years in history, the door to a parallel world suddenly opens, and strange beings appear before a group of high-school students who had been living peaceful lives.

These students accept their fate — to become the miko and her guardians — and decide to fight in order to save their world.