It's Christmas time and the Griswolds are preparing for a family seasonal celebration, but things never run smoothly for Clark, his wife Ellen and their two kids. Clark's continual bad luck is worsened by his obnoxious family guests, but he manages to keep going knowing that his Christmas bonus is due soon.
On July 2, a giant alien mothership enters orbit around Earth and deploys several dozen saucer-shaped 'destroyer' spacecraft that quickly lay waste to major cities around the planet. On July 3, the United States conducts a coordinated counterattack that fails. On July 4, a plan is devised to gain access to the interior of the alien mothership in space, in order to plant a nuclear missile.