Lost Coast opens with the protagonist, Gordon Freeman, finding himself near a group of decaying piers, underneath a monastery set up on rocks and overlooking the small town of St. Olga. A fisherman recognizes Gordon and directs him to the monastery, which the enemy Combine are using as a platform to launch artillery shells filled with headcrabs into the town. The fisherman opens a gate, allowing the player to proceed, and awaits Gordon's return.

As Gordon proceeds up the cliffside to the monastery, he encounters heavy resistance from Combine soldiers who rappel down the cliff to try to stop him. Gordon fights his way up, enters the monastery's sanctuary, and disables the artillery launcher. This alerts nearby soldiers, who assault the sanctuary in force, while a Combine attack helicopter arrives to support the soldiers. After defeating the soldiers in the courtyard, Gordon moves to scaffolding over the side of the cliff and destroys the helicopter with RPGs. The helicopter crashes into the scaffolding, freeing up a path to a crude elevator which lowers Gordon back down to the pier. The fisherman congratulates Freeman on his success, invites him to a feast in St. Olga, and the screen fades out. As the level ends, the fisherman exclaims that Gordon is "getting all fuzzy "round the edges"...