Following the destruction of the Citadel in City 17, Gordon Freeman and Alyx Vance find themselves stranded in the surrounding countryside. Alyx carries vital intelligence stolen from the Combine, data that reveals the location of the Combine's homeworld and could allow the human resistance to shut down a superportal forming above the ruins of the city. Pursued by Combine forces desperate to prevent the information from reaching the resistance base at White Forest, the pair must cross dangerous terrain filled with hostile creatures and enemy patrols to deliver the data before the portal fully opens and enables a massive Combine reinforcement.