Magic was once strong in Granvallen, the miraculous world of floating continents. A thousand years ago, six demons lords rebelled against the benevolent twin goddesses Aplyes and Espina. Thus the Great Sorcery War began, in which humans fought valiantly against demons. Tiara, the Priestess Princess of the Holy Kingdom of Valence, and the Crusader Paradys managed to defeat the demons, but the force of magic that penetrated Granvallen began to fade as a result.
Five hundred years later, a mysterious masked man enters the peaceful village of Puck on the continent of Arshes. Two fourteen-year-old children, the girl Pipiro and the boy Pokkle, witness how he enters the village temple, murders a guard, and steals six Goddess statues, the village's biggest treasure. The kids decide to leave Puck, track down the stranger, bring him to justice, and perhaps gain fame and fortune in the process.
Zwei!! is an action role-playing game with bright, cartoon-like 2D visuals. The player can switch between Pipiro and Pokkle at any time; when controlling one character, the other is handled by the AI. Pipiro uses magical projectiles while Pokkle fights with a drill-like close-combat weapon. Elemental and combo attacks can be used to exploit enemy weakpoints and force them to drop better items.
The player accesses individual locations from the overworld. Most of them are labyrinthine dungeons with multiple levels populated by enemies. Experience points are obtained from food dropped by enemies or found in the game world. Food items can be combined in a restaurant to create more powerful ones with a higher experience count. The player can also acquire a pet animal (a cat or a dog), who follows the protagonists, helps them collect items, and unlocks specific combo attacks.
Zwei!! contains various in-game items that serve to unlock mini-games and accessories. A shooting game entitled Zwei!! Shooting is accessed from within the game; the others can be launched from the desktop when the main game is closed. One of these, Pet Monitor, is obtained automatically and allows the player to make choices within an adventure-like environment, which may increase the pet's level and reward items in the main game. Other two mini-games are the typing tutor Typing with Ys, starring characters and themes from Ys games, and the puzzle game Mona Mona. Accessories include a calendar, a clock, and a calculator.