Commander Keen 2: The Earth Explodes (1990) is the second game in the Commander Keen saga. In it, the Vorticon mothership intends to destroy Earth, and only eight-year old Billy Blaze, as Commander Keen, can save us.
Here the author beats back the Vorticon invasion in slightly under 5 minutes, leaving one to wonder why we here on Earth bother with military defense at all. It's not quite that easy though; reading the author's comments is recommended to understand the planning and manipulation required for Keen to waltz by Vorticon after Vorticon.