Zerø is the fourth entry in the 4th Unit series and a direct sequel to Dual Targets. Only a few days have passed since the andoird assaults and the shocking encounter with Blon-Win's clone. The Unified Force gathers all its resources, preparing to engage the shady trade organization WWWF. However, a strange accident leads Blon-Win to assume that one of the Unified Force's members might be a traitor.
Gameplay-wise, Zerø returns to the Japanese adventure format of the first two entries, abandoning the visual novel style of its immediate predecessor. However, it still retains the emotional response system (only three out of five are present: "nice", "angry", and "sad"), which is sometimes needed to steer conversation. Topics are fixed for all dialogues. Much of the game involves looking for people located in various areas of the Unified Force base, trying to figure out who the spy is. The simple turn-based combat is present in this installment as well. The player can choose to attack any person Blon-Win encounters; however, attacking an ally would result in a Game Over regardless of the battle's outcome.