Serious Sam HD The First Encounter is a first-person shooter video game developed by Croteam and published by Devolver Digital. The game takes place after the events of Serious Sam 3 BFE and the introduction reveals what happened before the actual story took place. In ancient times, Earth was involved in a massive conflict between Mental, an evil extraterrestrial being who wants to rule the universe, and the Sirians, a technologically advanced sentient alien race that left many of its artifacts to be discovered by humanity. In the 22nd century, Mental and his army of grotesquely twisted monsters return to Earth intending to destroy humanity.

The humans are quickly overwhelmed. As a last resort, humanity uses the "Time-Lock". This mysterious Sirian artifact contains the power to transport a single person back to a chosen point in time. Because of his bravery in fighting these monsters, Sam "Serious" Stone is chosen to use the Time-Lock in hopes that he will defeat Mental and change the course of history.