“To rescue his parents and save humanity, Renaissance artist Leonardo Da Vinci must destroy an intergalactic megalomaniac’s alien armada with his unproven inventions.”

Maybe the joke is that the genius Leonardo Da Vinci is really a drunkard and the alien side-kick is the one who did all the "discoveries" and talented work.
The mysterious smile may be due to the fact that Alien Mona Lisa like Vincent d'Onofrio character in Men In Black is a skin crawler.

I think you have burried the story....

Leonardo DaVinci must use his .... to save his parents and humanity from a maniacal alien.

You should give him a friendly alien; disguised as a human, as a sidekick (This alien would, of course, be Mona Lisa)