On July 2, 1996, an enormous mothership orbits the Earth, deploying multiple saucers, each fifteen miles wide, that take positions over Earth's major cities. David Levinson, an MIT-trained satellite technician, decodes a signal embedded within global satellite transmissions, realizing it is the aliens' countdown timer for a coordinated attack. With help from his ex-wife, White House Communications Director Constance Spano, David and his father Julius gain access to the Oval Office and alert President Thomas Whitmore to the threat. Whitmore orders evacuations of New York City, Los Angeles, and Washington, D.C., but it is too late. Each of the saucers fires a single destructive beam, causing a gargantuan explosion that incinerates each saucer's targeted city, killing millions. Whitmore, the Levinsons, and a few others escape aboard Air Force One as the capital is destroyed along with New York, Los Angeles, and other major cities around the world.
On July 3, counterattacks against the alien invaders begin, but their ships are shielded by force fields. Each saucer launches a swarm of fighter vessels with their own shields, which decimate the human fighter squadrons and military bases. Captain Steven Hiller, one of the mission's squadron leaders, survives by luring his attacker to the enclosed spaces of the Grand Canyon before ejecting from his plane, causing the enemy fighter to crash-land. He subdues the downed alien and flags down a convoy of refugees, hitching a ride with former combat pilot Russell Casse. They transport the unconscious alien to Area 51, where Whitmore's group has landed. Secretary of Defense Albert Nimzicki has revealed that a faction of the government has been involved in a UFO conspiracy since 1947, when one of the invaders' attack fighters crashed in Roswell. Area 51 houses the now-refurbished ship, and three alien corpses recovered from the crash. As scientist Dr. Brackish Okun examines the alien captured by Steven, it regains consciousness and attacks, telepathically invading Okun's mind and killing other personnel. It uses Okun's vocal cords to reject Whitmore's offer of peace before launching a psychic attack against him. After Secret Service agents and military personnel kill the alien, Whitmore reveals what he learned when they were linked: the invaders plan to strip Earth of natural resources. He reluctantly authorizes a nuclear attack against one warship situated above Houston as a trial, but it remains intact. Steven's fiancée Jasmine Dubrow and her son, having survived the destruction of Los Angeles, rescue a truckload of other survivors, including the critically injured First Lady, Marilyn. Though they are later found by Steven and taken to Area 51, Marilyn dies shortly after being reunited with her family.
On July 4, David writes a computer virus that will disrupt the aliens' shields' operating system, and devises a plan to upload it into the mothership from the refurbished alien fighter, which Steven volunteers to pilot. The U.S. military contacts different nations' surviving airborne squadrons around the world through Morse code to organize a united counter-offensive against the aliens. With military pilots in short supply, Whitmore and General William Grey enlist the help of volunteers with flight experience, including Russell, to fly the remaining jets at Area 51; and the former leads an attack on a warship bearing down on the base while the latter oversees it. Entering the mothership, Steven and David upload the virus and deploy a nuclear missile on board, nullifying the aliens' communications system and reinforcements after the ship is destroyed. With the aliens' shields deactivated, Whitmore's squadron can combat the enemy fighters, but their ammunition is exhausted before they can destroy the saucer. As it prepares to fire on the base, Russell has one last missile, but is unable to fire due to a launcher malfunction; he sacrifices himself by crashing his jet with the missile into the saucer's weapon, which destroys the warship. Human military forces worldwide are notified of the alien ships' critical weakness and destroy the others. As humanity rejoices in victory, Steven and David return to Area 51 and reunite with their families.
Will Smith as Captain Steven Hiller,
Bill Pullman as President Thomas J. Whitmore,
Jeff Goldblum as David Levinson,
Mary McDonnell as First Lady Marilyn Whitmore.
Judd Hirsch as Julius Levinson,
Robert Loggia as General William Grey, USMC,
Randy Quaid as Russell Casse,
Margaret Colin as Constance Spano,
Vivica A. Fox as Jasmine Dubrow,
James Rebhorn as Albert Nimzicki,
Harvey Fierstein as Marty Gilbert,
Adam Baldwin as Major Steven Mitchell, USAF,
Brent Spiner as Dr. Brackish Okun,
James Duval as Miguel Casse,
Bill Smitrovich as Lt. Colonel Watson.
Harry Connick Jr. as Marine Captain Jimmy Wilder,
Mae Whitman as Patricia Whitmore,
Ross Bagley as Dylan Dubrow-Hiller,
Lisa Jakub as Alicia Casse,
Giuseppe Andrews as Troy Casse,