Here is an overview of the game, via its Steam page:
Humanity is under siege as the full force of Mental’s hordes spread across the world, ravaging what remains of a broken and beaten civilization. The last remaining resistance to the invasion is the Earth Defense Force led by Sam “Serious” Stone and his heavily-armed squad of misfit commandos.
Croteam returns with a high-powered prequel to the Serious Sam series that scales up chaos to unprecedented levels. The classic Serious Sam formula is revamped by putting an unstoppable arsenal up against an unimaginable number of enemies that requires players to circle-strafe and backpedal-blast their way out of impossible situations.
- Hordes of Invaders – The iconic cast of alien invaders returns with some new reinforcements! Fight your way through unbelievable numbers of Mental’s minions including the iconic Headless Kamikaze, Beheaded Rocketeer, Kleer, Scrapjack, Werebull, and Khnum! Square off against brand new enemies and towering monsters like the frantic Processed, repulsive Belcher, the hard-hitting Zealot, and more.
- Explosive Arsenal – Armed with a slew of devastating weapons, pick your tool for any situation. Lay waste to Mental’s Horde using the powerful double-barreled shotgun, the punishing minigun, the powerful chainsaw launcher, a brand new auto shotgun, and the iconic cannon. Upgrade your toys, and enjoy the violent beauty of the lock-on rocket launcher, and the mighty laser beam of death.
- Cooperative Mayhem – Smash through the action-packed campaign with friends in four-player online co-op mode! Tackle exciting primary missions and thrilling side quests in modified difficulties for an extra challenge.
- Legion System – Serious Sam 4 unleashes some of the biggest moments in the series’ history with the new Legion System and battlefields teeming with thousands of enemies!