Here's what I think of the story so far:
Gordon Freeman, and average man, gets hired at the Black Mesa Research Facility. On one of his early days on the job, he has to put on a HEV suit and put a crystal sample from an alien world. When he puts it into the machine, everything goes wrong an explodes, but he is saved because for a brief moment, he is inside the world Xen. When he comes back all hell has broken loose. He fights his way past the aliens and soldiers while the Gman watches him. Along the way, people are telling him to get to the Lambda Complex, because the science team there can solve the problem. When he finally gets there, he gears up and gets ready to go into Xen, where he needs to kill the leader of the aliens, Nihilanth. He makes his way through Xen and finally gets to Nihilanth. Nihilanth mumbles about what seems to be the Gman before Gordon kills him. When Nihilanth finally dies, Gordon gets transported to a stange series of enviroments that mix Xen and Earth. Gman offers him a job, or a battle he can't win. According to HL2, Gordon accepts and steps into the portal, which leads to HL2...
What Blue Shift tells us about the story:
Next subject of the story, is Barney Calhoun. Barney has to escape the facility, and starts at the same time as Gordon. He finds Dr. Rosenberg, who leads him to a run-down part of the facility. Barney finds out that this was a pre-Lambda section, and the equipment inside it was used to transport HEV scouts to Xen to retrieve samples. The transport machine on Earth was dependent on a machine on Xen, so Barney had to go activate that machine. He did, and when he came back, the machine on Earth needed a new battery, so Barney goes down into the basement to get one. The military are invading the facility as he goes down, and he encounters some of them down there. He gets a battery, sends it up, and goes back up himself. The new battery is installed and the science team that was there all transport to the outside. Barney barely makes it, but when he does, he finds that he's stuck inside a "resonance cascade." He finally gets out and Dr. Rosenberg and the others drive away.
What OP4 tells us about the story:
The military is invading the facility. Now you are one of the soldiers. Corporal Shepard. Shepard and the others don't know what you're supposed to do there, but shortly after the crash, Shepard finds out that you're there to kill everyone, but you have a more important mission: to escape alive! Shortly before Gordon goes to Xen, new, tougher aliens from Xen start appearing. Shepard starts to see that Earth and Xen materials are combining, there is Xen land everywhere. Shepard finally gets to a strange room where you battle a huge alien. When Shepard defeats him, he is transported to the Gman, where he talks to Shepard about sparing him. During the talk, the bomb goes off, and Black Mesa is destroyed. Gman steps into a portal, and Shepard is all alone...