Violet Detector, like its predecessors Shoot the Bullet, Double Spoiler and Impossible Spell Card, is a collection of many individual danmaku patterns that the player can choose from at their leisure. In addition to the usual mechanic of taking photographs that clear enemy bullets, Sumireko has several psychic superpowers at her disposal, such as teleportation. The game features all of the characters who debuted in Legacy of Lunatic Kingdom and Hidden Star in Four Seasons, plus Reimu, Marisa, and most of the final and EX bosses from previous games.

Sumireko Usami, psychic teenager and frequent visitor to Gensoukyou, finds herself trapped in a nightmare wherein Gensoukyou's residents are relentlessly attacking her with danmaku. A frustrated Sumireko decides to make the best of the situation by photographing the danmaku patterns to share on social media, and after doing so, finds that she's been freed from the dream. Three long weeks of danmaku nightmares follow...