Once upon a time, there was a world where far too many kings made promises and would not keep them. Your name is Tom Thumb. You and your younger sister Anna have made it your mission to help these kings and get rid of those who want to claim the royal promise.

The Promise is an 18-card boss battler in the Grimm World universe. You battle with two characters against one enemy. Because your characters are very little, enemies are rats or birds. After hunting them, you take out the animals and build new equipment from their remains. This allows you to hunt these animals in a higher level, which will bring you more and also better body parts to craft more powerful weapons and armor.

In the Promise you have 5 battles in which you can hunt two different animals in 3 different levels. After that you will face the final boss: Hans the Hedgehog. If you win against him, you'll cancel the promise and win the game.