You are attempting to deliver Smart Drugs to Central Knot City, On the way Sam (Played By Norman Reedus) loses his Motorcycle and you must walk, You get caught in Timefall when you decide to enter a Cave, In the Cave you meet "Fragile" played by Lea Seydoux.

She helps you to not get sucked up by the BTs (Beached Things). Once you deliver the Smart Drugs you go on a Corpse disposal mission where you get caught in Timefall with BTs, You see a mysterious Man with a Golden Mask who appears to make the BTs attack you.

One of the other members throws his BB pod (Bridge Baby) to you before he gets sucked up, this causes a void-out where you go to a Underwater realm and recover your body, You awaken in a Bridges facility with Blue Cuffs where you meet Deadman.

Source: https://deathstranding.fandom.com/wiki/Prologue:_Porter