The game itself started with a set of plant in your side battling with other zombies in the ground. The battle system is automatic and set.
After each battle your plants in your side gets experience and leveled up together with it you get some rewards. Player doesn't get experience after each battle, player gain experience during the process of photosynthesis for your plants such giving water or fertilizer to it. The process for photosynthesis allows plants to recover its health points.