Short version: HL-2 does not owe a lot to HL-1 except for the hero and a few minor characters. Here’s my synopsis of HL-1 for what it’s worth:
Half-Life (1): Dr. Gordon Freeman is a physicist working for the United States government at the Black Mesa complex, a former missile base converted into a classified research lab, located in the American southwest. Dr. Freeman is a newcomer and doesn’t have full security clearance yet, but his job involves work in the Anomalous Materials section of the facility, analyzing bizarre substances of unknown origin. He has just finished qualifiying for use of the Hazardous Environment Mark IV suit, a prosthetic body armor and protective gear used at Black Mesa.
At the beginning of the story, Dr. Freeman reports to Anomalous Materials to take part in an important experiment. The “largest and purest sample yet” of a strange glowing crystalline substance is to be subjected to analysis using the Anti-mass spectrometer. Despite warnings that their equipment is already overloaded, and that theoretically a “resonance cascade” reaction could occur, the plan is to run the spectrometer past it’s normal limits in order to get definitive results. Dr. Freeman’s role is decidedly low-tech: wearing the protective HEV-IV, he will be in the test chamber during the experiment to manually insert the sample into the particle beam. The moment that he does, all hell breaks loose. Whether it’s an accident or whether the sample was sabotaged, no one knows. But as soon as the sample hits the beam, uncontrolled energies begin to be released. Freeman finds himself briefly teleported somewhere else, glimpsing a strange alien world before blacking out.
When Freeman regains conciousness, he is back in the now ruined test chamber, with massive structural damage is everywhere. Struggling to climb back up to the main facility, he encounters hostile alien creatures that appear out of nowhere in flashes of green light, as well as former humans now serving as the hosts of alien parasites. Freeman struggles to work his way through the ruined facility, bypassing blocked corridors and collapsed elevator shafts to get to the surface and get help. He is aided by other survivors he encounters and acquires weapons to fight off successive waves of new hostile aliens. He also catches glimpses of a mysterious figure, a government official known only as the Administrator, following his progress.
Finally near the surface, Freeman recieves word that government soliders are on their way to secure the facility. But when Freeman encounters them, he discovers to his horror that they have orders to “sterilize” Black Mesa, killing all witnesses as well as the aliens. Now Freeman has to fight against his fellow man as well as the aliens. He reaches the surface only to discover that escape is impossible; the facility is surrounded by government forces shooting at anything that moves. Retreating back underground, Freeman is told by a surviving scientist that the only possible salvation lies with the “Lambda Complex” group, on the opposite side of the base.
To reach the Lambda complex, Freeman must make his way through deep underground areas consisting of a rocket test lab, an underground tramway, a satellite launching facility, a flooded section, and various areas devoted to industrial machinery and freight storage areas. All the time the alien attacks increase as well as the government’s more and more drastic countermeasures.
Finally reaching the Lambda Complex, Freeman discovers that it is devoted to experiments in teleportation. The surviving scientists there explain that they had discovered a means of teleportation based on transferring into another dimension called Xen, the home universe of the invading aliens, who are now countering the intrusions into their world with an invasion of their own. The only possible way to stop them is to send someone on a suicide mission into Xen to find the energy source powering the invasion, and destroy it. Freeman volunteers.
In the bizarre universe of Xen, Freeman navigates through multiple teleportation sites, fighting against the inhabitants to remain alive, while homing in on the energy source. He finally finds the master intelligence controlling the invasion, and after a tremendous battle kills it, ending the menace to Earth.
In the epilog, Freeman is rescued by the sinister Administrator, who apparently works for some agency with vastly more power and knowledge than the US government. He offers Freeman a choice: certain death, or to work for the Administrator’s mysterious superiors as an expendable agent.