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 through time a single individual who can, hopefully, defeat Mental and alter the course of history. Because of his bravery in fighting the monsters, Sam "Serious" Stone is chosen to use the "Time Lock".