The game opens up with a intercom introducing you to Black Mesa as you come in through the Black Mesa tram station, The game introduces you as Gordon freeman as you witness how big Black Mesa truly is (It's theorized to atleast be 100 km). The tram goes through multiple different enviorenments such as eating quarters, labs, etc. You even get to see another guard also late! (Foreshadowing). Gordon finally gets to his sector of the Black Mesa facility, Anomalous Materials. The sector consists of testing on supernatural or weird things such as portals. Mostly to rival with their competiter, Apeture Science (This comes back quite a bit, and also is the main lab you are in when playing Portal 1/2.) Gordon throws on an HEV suit, This suit allows him to stay protected and be used as armor. After getting the HEV suit he makes it to the test room as Black Mesa has finally finished with their portal to another alien planet, also known as XEN (Spelt like ZEN). As the portal opens he puts a mysterious crystal into it that was donated by a mysterious gifter in a suit, this would cause Unforseen Concequences as the lab goes into a cascades event causing aliens to invade and start destroying everything. Gordon wakes up and finds a crowbar, fighting through aliens such as Headcrabs (aliens that jump onto the head to turn the host into a zombie) or vordiguants (Aliens prisoner of the Nihilanth, forced to fight humans.) As gordon fights for his life, while also saving many more, Gordon gets word that military forces are sent to "help" every evacuate and get rid of the aliens. But gordon soon witnesses a HECU soldier (HECU are the military forces) shoot one of the few remaining scientists throughout Black Mesa. Gordon begins now not only fighting aliens, but also the maniac HECU soldiers running loose around the facility. Gordon soon gets to a point where the HECU detect him as a threat, and he is targeted. But soon enough, the HECU numbers start to plumet, The aliens, And gordon, push the HECU into evacuating themselves leaving many behind. Gordon finds some remaining scientists and they are able to open the portal to XEN so gordon can stop whatever is causing the aliens to keep coming. Gordon Finally makes it to xen, having a last stand against the remaining aliens there and soon he meets the Nihilanth. The Nihilanth warps portals into reality, sending them towards gordon as you must dodge them and shoot the head of the Nihilanth. Finally theres a gap in the Nihilanth's head and you can shoot it, ending it and destroying the only "Leader" of XEN. Gordon passes out, but soon awakens to a man with a suit. He recaps what gordon has done, such as stopping the HECU and preventing XEN from coming that much anymore. The man offers a job opening for gordon, although not stating exactly what the job is, it's life or death for Gordon. Gordon accepts the deal, being put in stasis in the process, Concluding the first game.
Half life doesn't have much reason, but the little reason it has revolutionized first person shooters (as said acouple times before), Valve not only wanted to make a amazing first person shooter, but also a compelling story that doesn't exactly tell the player, but shows the player through its many environments such as Black Mesa and its final collapse. The beginning of the game shows this very well, with an operating railway system you get to see most if not all of Black mesa before the Cascades Event.
Half life has been out for 27 years, throughout it's early development it had many MANY scrapped content, with that being said here is what has been scrapped:
Mr. Friendly was supposed to be more of a stalker type enemy, not exactly attacking but hiding away from the player to find dead bodies to clean up from the map, Mr. Friendly was supposed to take the bodies and improve preformance while scaring the player when spotted. But the design of the alien was rather controversial so it was cut.
This was supposed to be an underwater enemy that could spit projectiles towards the player, but was soon scrapped and replaced by the Ichthyosaur.
This was supposed to be a fast and agile enemy going faster than any other enemy in the game, It would hunt in dark areas and was even shown in the early alpha demo before being scrapped for unknown reasons.
This was supposed to be a passive balloon like alien that would just float around xen, reason for being scrapped are unknown.
This was suppsoed to be a small insect that would be used by the player to attack other aliens, soon replaced by snarks.
This was supposed to be a small six eyed toad that would lead aliens towards certain areas used by the player, reasons for being scrapped unknown.
This was supposed to be a seperate variant of the glock you got in the game but with a suppresor so you could not be heard while shooting, reasons for being scrapped unknown.
This was supposed to be a weapon used to burn enemies and light up dark areas, reason for being scrapped unknown.
There is tons of different scrapped ideas in this game, but I can't exactly name them all so that will conclude this.