Twenty years prior to the story's beginning, an asteroid headed for Earth destroys Pluto. Due to Pluto's destruction, the asteroid, which is dubbed Thanatos, becomes temporarily stagnant. Now, in order to avoid Earth's impending destruction, the International Solarsystem Development Agency (ISDA) works on the "D-Project", and creates the "Dragonaut" after finding a dragon egg under the ocean. This weapon's primary purpose is to destroy the asteroid when the time comes. (myanimelist.net)