The HL Invasion scenario starts at the end of Half-Life. Remember, you were in a train with the strange G-man ... Fortunately, scientists hidden in a subsidiary laboratory with Black Mesa manage to repatriate you there with a space probe, in which you jumped just before the G-man kills you. Once awoken, you discover this secret laboratory that the government forces haven't discovered yet. You are watching a teleportation experience, when suddenly overtrained marines invade this hiding place ... From this moment, you'll never be alone ... This new trip will lead you into subsidiary areas with Black Mesa where you'll get more revelations about the government objectives and the world of Xen ...