In ancient times, Earth was involved in a conflict between Mental and the Sirians, an alien race that left many of its artifacts to be found by humanity. In the 22nd century Mental's forces return to Earth hell-bent on eradicating humankind: as a last resort the usage of the "Time Lock" is decided: this Sirian device can send back a single individual through time. Because of his bravery in fighting the monsters, Sam "Serious" Stone (who is chosen to use the "Time Lock") can hopefully defeat Mental and alter the course of history. As the game opens, Sam appears in Ancient Egypt. He travels from one Egyptian monument to the next, fighting against the monsters Mental sent after him along the way.