When timid bank clerk Stanley Ipkiss discovers a magical mask containing the spirit of the Norse god Loki, his entire life changes. While wearing the mask, Ipkiss becomes a supernatural playboy exuding charm and confidence which allows him to catch the eye of local nightclub singer Tina Carlyle. Unfortunately, under the mask's influence, Ipkiss also robs a bank, which angers junior crime lord Dorian Tyrell, whose goons get blamed for the heist.
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.
Obsessive master thief, Neil McCauley leads a top-notch crew on various daring heists throughout Los Angeles while determined detective, Vincent Hanna pursues him without rest. Each man recognizes and respects the ability and the dedication of the other even though they are aware their cat-and-mouse game may end in violence.