Code Vein is an action role-playing game set in an open world, post-apocalyptic dystopian environment and played from a third-person perspective. Its gameplay is inspired by the Dark Souls series of games, and the animation is inspired by God Eater 3, also published by Bandai Namco.

Set in the near future, the world falls to a mysterious calamity known as the Great Collapse with humanity weaponizing their dead into a vampiric fighting force known as Revenants to fight monsters that began emerging around the world. Brought back to life when a BOR parasitic organism is attached to the heart, a Revenant is brought back to life at the cost of random memories and can only die if their heart is destroyed. Revenants also require human blood as nourishment to keep themselves from entering a frenzy and mutating into the Lost – cannibalistic beasts that the other Revenants hunt and execute. Cruz Silva volunteered for the experiment and became the Queen of the Revenants to stabilize the Revenants before she became frenzied herself, causing a high death toll of the surviving human population. More Revenants were produced to contain the damage in a mission called Operation Queenslayer led by Cruz's father Gregorio Silva, a charismatic military leader who established Vein's provisional government. Though they succeeded in killing Cruz, the situation worsens with Bloodspring plants slowly dwindling in their ability to produce nourishing Blood Beads as an alternative to human blood, with miasma hastening the Revenants' bloodthirst and turning frenzied, unless wearing filtration masks which are mostly effective when miasma concentrations are low.