Taken from OneManga:

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 in Pluto's orbit. Now, in order to avoid Earth's impending destruction, the International Solarsystem Development Agency (ISDA) works on the "D-Project", and secretly creates weapons called "Dragons" after finding a dragon egg under the ocean. However, they soon find out that the asteroid is not their only threat, as powerful, destructive dragons from Thanatos appear on Earth.