Something is living in the Earth's core, and it is kept trapped inside by the Earth's own magnetic field. It was trapped there billions of years ago before life existed on the planet. But now, as the magnetic field is failing, and the creature held inside it is threatening to escape. If it does, it can wreak havoc across the galaxy.
The planet must be destroyed for the good of the galaxy. But how can one condone the destruction of the Earth if it means the genocide of the human race?