***SPOILER WARNING***
This is my interpretation of the Half-Life story from the beginning until the end of Half-Life 2.
Begin Half-Life 1 (Black Mesa)
So you just got your degree in theoretical physics, and you're off to another day at your job in some huge complex. You really don't know what the experiments that you perform are used for, but you do it anyway. One day an experimentation that you perform creates a certain portal that allows aliens from the planet Xen access to Earth. You fight your way to the top, through aliens and marines. After which you are met with a strange suited man simply called the "G-Man." My guess is that the G-Man (and whatever organization he works for) was monitoring the situation at Black Mesa after learning that the administrator was up to no good. He views your progress throughout your endeavor, and realizes that you are the right man for what he does.
So what exactly does the G-Man do? Keep reading and I'll explain in detail.
Begin Half-Life 2 (City 17)
Now we're employed by the G-Man to stop yet another threatening situation. He has trust that you can handle it (you are Gordon Freeman after all). One interesting thing that the G-Man tells you is, "...and all the effort in the world would have gone to waste, if it wasn't for...well let's just say your time hasn't come yet." I'm guessing that the effort he's referring to is his organizations efforts at stopping Dr. Breen from succeeding in aiding an alien invasion of earth. This can be assumed from the ending of Half-Life 2 when you are traveling up the elevator with Alyx. You overhear Dr. Breen speaking with an alien (Combine? Xenian?) about "handing over Earth." I like to think that Dr. Breen was paid to allow a portal from Xen to Earth (in Half-Life 1) so that they could take control and possibly evolve humans into their kind (Nova Prospekt?)
OK, so you're working for the G-Man in trying to stop a possible alien evolution of the planet Earth...so why are the Vortigaunts hiding out with the other rebels? After talking to the alien who mounts the gun to your boat, I have come to some conclusions. He refers to Eli Vance as "the one who works for liberation of his people." I guess these Vortigaunts were slaves to the Xen population and want to be liberated (assumed from the Xen scene in Half-Life 1 when the aliens are seen working on unknown things in the alien factory). This alien also refers to "The Freeman" as the one who cut the cord of life from the Nihilanth essentially giving new life to the Vortigaunts.
In Summary
The G-Man works for an organization that is attempting to stop Dr. Breen's actions, as well as the Combine and Xen (are they related?) from changing the way Earth is (if it is Earth). He spots you in Half-Life 1 as a man who could potentially turn the tides after the alien invasion of Black Mesa. You are further employed by him to carry out his duties against the Combine in Half-Life 2. He supplies you no information or weapons, and fully assumes you are capable of finding a way to take down Dr. Breen and his administration. As for Dr. Breen, he is helping aliens in evolving and taking over the planet Earth. However, Dr. Freeman happens to have plans that differ from those of the administrator. You are after all, the right man in the wrong place, and you definitely made all the difference.
Again, this is just my interpretation, as I love filling in the blanks how I see fit. If anyone wants to add their thoughts, let's make a good discussion of this thread.
This is my interpretation of the Half-Life story from the beginning until the end of Half-Life 2.
Begin Half-Life 1 (Black Mesa)
So you just got your degree in theoretical physics, and you're off to another day at your job in some huge complex. You really don't know what the experiments that you perform are used for, but you do it anyway. One day an experimentation that you perform creates a certain portal that allows aliens from the planet Xen access to Earth. You fight your way to the top, through aliens and marines. After which you are met with a strange suited man simply called the "G-Man." My guess is that the G-Man (and whatever organization he works for) was monitoring the situation at Black Mesa after learning that the administrator was up to no good. He views your progress throughout your endeavor, and realizes that you are the right man for what he does.
So what exactly does the G-Man do? Keep reading and I'll explain in detail.
Begin Half-Life 2 (City 17)
Now we're employed by the G-Man to stop yet another threatening situation. He has trust that you can handle it (you are Gordon Freeman after all). One interesting thing that the G-Man tells you is, "...and all the effort in the world would have gone to waste, if it wasn't for...well let's just say your time hasn't come yet." I'm guessing that the effort he's referring to is his organizations efforts at stopping Dr. Breen from succeeding in aiding an alien invasion of earth. This can be assumed from the ending of Half-Life 2 when you are traveling up the elevator with Alyx. You overhear Dr. Breen speaking with an alien (Combine? Xenian?) about "handing over Earth." I like to think that Dr. Breen was paid to allow a portal from Xen to Earth (in Half-Life 1) so that they could take control and possibly evolve humans into their kind (Nova Prospekt?)
OK, so you're working for the G-Man in trying to stop a possible alien evolution of the planet Earth...so why are the Vortigaunts hiding out with the other rebels? After talking to the alien who mounts the gun to your boat, I have come to some conclusions. He refers to Eli Vance as "the one who works for liberation of his people." I guess these Vortigaunts were slaves to the Xen population and want to be liberated (assumed from the Xen scene in Half-Life 1 when the aliens are seen working on unknown things in the alien factory). This alien also refers to "The Freeman" as the one who cut the cord of life from the Nihilanth essentially giving new life to the Vortigaunts.
In Summary
The G-Man works for an organization that is attempting to stop Dr. Breen's actions, as well as the Combine and Xen (are they related?) from changing the way Earth is (if it is Earth). He spots you in Half-Life 1 as a man who could potentially turn the tides after the alien invasion of Black Mesa. You are further employed by him to carry out his duties against the Combine in Half-Life 2. He supplies you no information or weapons, and fully assumes you are capable of finding a way to take down Dr. Breen and his administration. As for Dr. Breen, he is helping aliens in evolving and taking over the planet Earth. However, Dr. Freeman happens to have plans that differ from those of the administrator. You are after all, the right man in the wrong place, and you definitely made all the difference.
Again, this is just my interpretation, as I love filling in the blanks how I see fit. If anyone wants to add their thoughts, let's make a good discussion of this thread.