Monsters starts with a NASA deep-space probe crash landing in Mexico. After the crash, extraterrestrial creatures that are aboard the spaceship start invading the border regions of Mexico and the United States. The military quarantines the northern half of Mexico as they try to fight the aliens.
Meanwhile, an American photojournalist named Andrew Kaulder hired by his boss to search for his daughther, Samantha Wyden. His investigation leads him to a Mexican hospital where he finds Samantha. Together, the two prepares to get back to the United States via a train. They discover that the rail tracks are already destroyed and they only have a few days to leave before they are caught by the quarantine. To avoid entrapment, the pair head to the border on foot.
Andrew makes out with a girl who steals their passports and Samantha’s ticket for the ferry. Samantha has no choice but to pay her fare with her engagement ring.
A riverboat manned by army escorts carry them to the Mexico-US border. The boat is attack by creatures. No one survives except Andrew and Samantha. They continue their journey and they find other dead bodies along the way. Just as they reach the border, the creatures have broken through. They push their way along the now empty Texas and end up in an abandoned gas station where they contact the army to rescue them.
As they wait for rescue, the pair contact their families as well. The two notice a creature sneaking up on the station attracted by the glaring screen of the television. Samantha unplugs the TV. The creatures leaves and encounters another of its kind. The two beings seem to communicate using light impulses. The military arrives after the creatures leave. The film ends where the invasion has started, when a creature attacks a military rescue team.