Think of it from Gordon's point of view. He went to work for a fairly routine experiment hen he's teleported to Xen, he's zapped back to the lab wich is now overrun with aliens. He fights for his life and tries to reach the surface and when he does they start shooting him and his fellow scientists. He makes his way passed the US military and an army of aliens to get teleported back to Xen. There he makes his way through floating islands and slave fectories to fight Nihilanth. And then he's locked up in stasis.
Seconds later (to him) he finds out he's 20 years in the future and a second alien invasion has taken over the earth, partly due to his actions back at Black Mesa. He's conscripted into the resistance by an old friend when the alien overlords decide to hunt him down. He evades Civil Protection through radioactive, zombie filled sewers. After shooting down a freaking helicopter and driving off a dam he reaches Black Mesa East and meets up with his old pal Eli. However, before he has time to rest the Combine assault BME and he is forced to flee to zombie filled Ravenholm. He and the preacher called Father Grigori fight through the town and Gordon decends into the mines. Here he fights through MORE zombies to reach the surface. He helps some resistance fighters defend against a Combine assault where they then give him a buggy. He takes the costal road, stopping by various resistance outposts. Eventually he reaches Nova Prospekt, where he intends to save the leaders of the Resistance, he meets up with Alyx Vance and they fight through the prison to rescue her father. Then they're betrayed by Mossman.
*gasp*
Gordon and Alyx take a teleporter to Dr. Kliener's lab, where he tells them they've been teleporting for a week and the raid on Nova Prospekt spurred the Resistance into rebellion. Gordon takes to the streets, along with D0g, to assist the Resistance and to reach Barney Calhoun. After plenty of street fighting Gordon meets up hith Barney and they assault the Combine supression field. Gordon then makes his way to the Citadel (with a lot of zombies and combine between him and there. He fights his way up the Citadel and is captured by Breen. his HEV suit protects him from Breen's makeshift execution, he persues Breen and destoys the teleporter at the top of the Citadel, prossibly killing Breen. EXPLOSION! G-Man places Gordon into stasis but leaves Alyx to die.
Or not! The Vortigaunts decide to step in, saving Alyxfrom the blast and pulling Gordon from stasis. The two end up at the bottom of the Citadel with D0g. Told via radio by Kliener that the Citadel will explode soon they make their way back inside to buy the Resistance some time to flee. Fighting off the frantic and confused Combine forces they reach the core and, through a series of jumping puzzles, contain the core, but only for a few hours. Deciding that hoofing it isn't the best option they instead decide to take a Combine train to the edge of the city. Unfortunately the train crashes into a collapsed part of the tunnels, forcing the duo to run out of the city. They join up with some resistance forces in the streets and meet Barney how is organising people into evacuation groups. Gordon and Alyx fight through a Combine controlled hospital in order to draw the Combine away from the evacuees before meeting them by the train station. They escort any survivors to the trains before climbing aboard the last train. EXPLOSION! Part deux!
Gordon wakes up aboard the now crashed train. He and Alyx decide to head to White Forest, the main Resistance base outside of City 17. They maketheir wy through some mines before Alyx is impaled by a Hunter and Gordon is knocked out, again. Luckily a Vortigaunt managed to find them before it was too late for Alyx. He takes gordon to a resistance crossing point, however Gordon fell into some antlion tunnels, where he had to fight endless waves of alien arachnids to eventually stumble across the crossing point. There, with Griggs and Sheckley, he fights off waves of Antlions defending Alyx until more Vortigaunts arrive. Then he travels into the Antlion hive to retrieve "the extract" which isused to heal Alyx. While she is being heale the G-Man rears his head and instructs Gordon to make sure Alyx survives. Alyx and Gordon find a car and drive it all the way to White For-nope, the engine broke down. They find a Resistance outpost and fight off ANOTHER Combine assault (these guys just don't give up). After clearing out a Combine outpost and having their car fixed Gordon and Alyx ride up to White Forest. After everyone is done with their greeting Gordon is sent to investigate some pigeons, howeer these areno ordinary pigeons. It turns out the pigeons were actually Combine pigeons! I mean soldiers! He fights off YET ANOTHER Combine assault when he's is informed of a massive Combine assault force on the way. Gordon gets a crash course in how to use Strider Busters and is sent off to save the day again. After finally fighting off the last of the Combine Gordon and the gang meet up to launch a rocket into space, as is the traditional celebration ceremony. The launch is a rousing success, sparking a possible upturn in Gordon's shitty luck streak. As he's told to go to Antarctica to find the Borealis some Combine Advisors bust in and eat Eli's brain (let's hope they don't gain his knowledge). The credits roll as Gordon is slumped against a wall and Alyx cries over he father's dead body.
And all that without any sleep, food or bathroom breaks!
The tragic tale of a brave scientist returning to work after Thyroid surgery. He should have stayed at home for further recuperation but he needed the money as he was being hounded by the worlds creepiest IRS auditor. 
