You will play as a young boy who's transported over a thousand years into the past when he stumbles across a magic artifact, and discovers he's the "Prophet", chosen to lead the Egg Knights into battle against the Dark Army. Most of your time in game is spent in battle, directing your Egg Knights against whatever monsters are currently attacking you.
When your Knight's attack bar fills up, you can click its portrait to attack whatever monster you're targeting. Each Egg Knight has a type that influences what it's strong or weak against... Wall types are weak against Siege, for example, and will deal less damage (and take more damage!) from Siege types.