In Kentucky, a family watches in confusion as a herd of cattle starts running past their home on fire. A spaceship is then seen departing Earth and returning to Mars, where it gathers hundreds of other Martian ships and heads back to Earth. President James Dale, along with his aides, addresses the United States concerning the historic event. The President's science aides set up a first contact meeting with the Martians in Pahrump, Nevada, as President Dale watches the development on TV with his wife Marsha and his daughter Taffy.
Using a translation machine, the Ambassador of Mars announces that they "come in peace". When a hippie releases a dove as a symbol of peace, the Ambassador shoots it before he and the other Martians slaughter most of the people at the event, including General Casey, news reporter Jason Stone and Billy-Glenn Norris, before capturing chat-show host Nathalie Lake and her pet Chihuahua.
Thinking that the Martians assumed that the dove was a symbol of war, President Dale tells Professor Donald Kessler to renegotiate with the Martians, whose ambassador later requests to address the United States Congress. At this meeting, the Martians massacre most of Congress. Kessler begs the Martian ambassador to stop, but is knocked unconscious and taken aboard their ship, where he is later shown with his body parts dismembered and his disembodied head remaining animated. General Decker tries to convince President Dale to retaliate with nuclear warfare, but the President refuses.
After a failed attempt to assassinate President Dale in which a disguised Martian is killed, the Martians invade Earth in droves, starting with Washington, D.C. and quickly spreading around the globe. As they attack the White House, the Secret Service evacuates the President, but the First Lady is crushed to death by the Nancy Reagan Chandelier and Taffy is separated from them during the chaos. After the President of France is assassinated by the Martians and the Martians have defaced a variety of world landmarks, the U.S. government attempts a nuclear attack on the Martian mothership that night, but that proves futile and the Martians continue destroying Earth and its inhabitants. Eventually, the Martian leader and his bodyguards breach the bunker where Dale has been taken and the former reduces Decker to the size of an insect before killing him. The Martians kill everyone else in the bunker except for Dale, who makes an impassioned speech in an attempt to plea for peace and his life. The Martian leader appears to be moved by the speech and deceptively appears to agree to a truce with Dale, but then uses a gadget disguised as a hand to kill him.
As the Martians ravage Las Vegas, Byron Williams, a former world champion boxer, leads a small group of survivors consisting of Barbara Land, Tom Jones, a waitress named Cindy and a lawyer to an airfield in the hopes of flying a small jet procured by Land to safety. They barely make it, losing the lawyer in the process, but discover a large group of Martians stationed there as they are preparing to take off, the ambassador among them. Byron creates a diversion by challenging them to a fistfight. While he succeeds in killing the ambassador, he is outnumbered and overwhelmed, but Tom, Barbara and Cindy escape.
Billy-Glenn's brother Richie discovers that the Martians' heads explode when they hear Slim Whitman's "Indian Love Call" when he goes to rescue his grandmother, Florence. Richie and Florence then drive around town, using the song to kill Martians, and broadcast the song on a local radio station. Thereafter, armed forces broadcast the song around the globe, killing the Martian leader and most, if not all, of the other Martians. Nathalie and Kessler's disembodied heads kiss while the Martian spaceship they are on crashes into the ocean.
In the aftermath, Richie and Florence are awarded the Medal of Honor by Taffy, presumably the only surviving member of the U.S. government. Byron, who survived the Martian brawl, arrives in D.C. to reunite with his former wife and their two sons as the devastation is being cleaned up. Barbara, Cindy and Tom Jones emerge from a cave with some ridiculously tame wild animals to see dozens of crashed Martian ships in Lake Tahoe. Tom Jones begins singing his signature song ″It's Not Unusual″.
