Presently, one of the greatest challenges of humankind is climate change. It is usually assumed that its mitigation will require an extensive adaptation in many human habits. As every fourth ton of carbon dioxide emitted can be attributed to the food chain, it is argued that one of the most effective actions an individual can take is changing towards a more sustainable diet, like a plant-based one. Serious games have proved to be a successful means for achieving behavior change. We present Veganity, your journey, a mobile serious game aimed at assisting players in their transition towards a more plantbased diet. The game combines the characteristics of the idle game genre with diet tracking: in the game, as you progress in adapting your personal diet, you spare energy that can be spent in making the game world more sustainable. Personal factors, like changing your ecological or social norms, your habits and your behavioral costs and benefits, are targeted by different game mechanics. These include, among others, a currency mechanism for stimulating habit change, in-game actions building up selfefficacy, and tips raising problem awareness. Besides describing our game design, we also present the main features of the game implementation. From a preliminary evaluation, we conclude that Veganity, your journey successfully engages players in pursuing their own diet goals, and increases their reflection on, and acquaintance with, sustainable diet behaviors.