President Scroob wants to get rid of his political rival, so he and Dark Helmet travel back to 1984 Earth so they can stop his family line.

Barf is bitten by a chest-bursting alien spider and becomes Spidermawg, just in time to save his home planet from the nasty no good Spaceballs. But President Scroob uses a new weapon to become the super powered Mothball as well.