What is the central plot of 'El Eternauta'?

The story follows Cornelio Fausto, 'El Eternauta' (The Eternal One), a man who can survive seemingly fatal injuries due to a unique physiological anomaly. He's tasked with investigating a mysterious phenomenon that causes people to revert to a primal state, transforming into savage creatures. The investigation leads him across a desolate, frozen Earth, confronting both the physical dangers and the psychological toll of his mission.