When we first meet Hiro Protagonist, he delivers pizzas for the mob in L.A.; he's also a brilliant hacker and sword-wielding genius, but his plays-poorly-with-others personality means he needs a day job. Luckily, when a skateboard courier called Y.T. saves his butt from a delivery-gone-bonkers, he decides to play nice with her. Watch out, A-Team, Team Awesome has officially been formed.
Why is it a good thing that Hiro and Y.T. are working together to gather information in both the real world and virtual-reality world called the Metaverse? There's a new drug named Snow Crash making the rounds, and it'll mess you up, whether you access it in the Metaverse or ingest it in meat-space. When one of Hiro's hacker buddies, Da5id, takes it and goes catatonic, the threat becomes personal. Also, Hiro's brilliant ex-girlfriend Juanita gets involved, spouting theories about early Christianity and ancient Sumer before she disappears.
Through a combo of intellectual sleuthing and dangerous expeditions, Hiro and Y.T. figure out that Snow Crash is tapping into some seriously dangerous and significant ancient stuff. Due to the structure of the human brain, people are susceptible to a metavirus that can essentially hack their brains and turn them into mindless minions. Even people who've significantly managed to rewire their brains by learning to code—i.e. hackers—can be infected by the computer-version of Snow Crash.
In the course of piecing all this together, Hiro runs into Raven, an Aleut assassin working for L. Bob Rife, a rich dude who's funding the research to promote and spread the virus. Hiro manages to escape with his life, but things just get more dangerous as he heads north up the West Coast to find Juanita, who hasn't been heard from since she went to Rife's Raft (a floating city built of cruise ships and refugee boats). Hiro teams up with the mafia to get onto the Raft. Much killing ensues.
Meantime, Y.T.—who has met and become a personal favorite of the mafia's head, Uncle Enzo—is kidnapped by the religious fanatics serving Rife. She winds up on the Raft, too, and is whisked away to its fancier parts by a mysterious suitor. Guess who that turns out to be? Raven. He's not so bad when he's not killing people.
Through a series of explosions and bad ideas, Hiro de-programs the people who'd had their minds wiped by Snow Crash, Y.T. rescues herself, Rife is killed, and Raven escapes. Hiro gets another chance with Juanita, and Y.T. works on patching up her relationship with her clueless mom. The world remains in a ridiculously dystopian state, but at least Snow Crash won't brain-drain everyone in the hands of a ruthless maniac like Rife.