Prison One

Approximately one hundred years ago, a group of aliens known collectively as Central Council built a prison within the Earth's crust so as to house prisoners from all over the galaxy.
One such prisoner was an emperor who had been leading an opposing group known as High Command.
Almost everyone on Earth is oblivious to the prison being there.
High Command also does not know of the prison's existence.
However, they are scouring the galaxy to locate the emperor and an energy source as their energy reserves are running critically low.
Because Earth was seen as a planet of interest, High Command had placed agents there who were locating clues and finding a girl in possession of an ancient alien artifact. They realize that this could very well be more of a place of interest than was first thought.
Central Council must now make sure the prison is secure and attempt to stop High Command from reaching Earth in a their new powerful space craft which they hope to use to affect a successful prison break.