An ancient meteorite crashes onto the Earth's surface. Six construction workers later begin work building an airstrip at the site, on an island off the coast of Africa.
Foreman Kelly (Clint Walker) and bulldozer driver Mack (Robert Urich) uncover the meteorite, which emits a strange sound. When the bulldozer (a Caterpillar D9) is used to try to shift the meteorite, it emits a blue light and seems to possess the bulldozer. Mack, standing nearby as this occurs, falls ill and then dies. Chub (Neville Brand), the team's mechanic, cannot find anything wrong with the bulldozer, but can hear the odd sound from the blade. Kelly orders that the bulldozer not be used.
Beltran (James A. Watson Jr.) ignores the prohibition and starts the bulldozer, bringing it to malevolent life. It destroys the camp's only two-way radio and begins a rampage, killing the workers one by one. It seems to run indefinitely in spite of a limited fuel capacity. The machine has some rudimentary intelligence and guile, and hunts down the men.
The crew is soon reduced to just Kelly and Dennis (Carl Betz). Running out of options they jokingly convict the bulldozer of murder and devise methods of 'executing' it. Too heavy to hang, too big for the gas chamber... until they realize it might be electrocuted. They lure it to a trap consisting of steel Marsden Matting (used for constructing temporary runways during World War II) connected to a generator.As the bulldozer is electrified, the alien entity emerges as an aura around the machine, then finally fades. The men shut down the power and check the blade: no sound. Kelly realizes his story will not be believed as he is a recovering alcoholic - and this job was his last chance to redeem himself.