As the mysterious and powerful "Guardian of Earth" your steely eye and hyper-powered laser arm have kept the citizens of Earth safe for generations (as well as proving a hit with the ladies).
With all that awesome talent and flair, surely it would be impossible for a devastating invasion of Doom Beasts to decend on your cities and cause havoc without your knowledge? (Even if you were maybe showboating for a magazine photo-shoot at the time...)
Now, you must not only defend your cities from the ravenous hordes of Doom Beasts - you must defend yourself against accusations of gross incompetence from the Galactic Counsel of Justice! And this is all before you can even start to get to the bottom or just who or what is behind the attacks...
Source: The Game Website.