OK,so a guy, a dude if you will, is bothered in his daily slackerhood by a couple of pretentious goons ( Thus always to deadbeats, Lebowski) who threaten him with a member of the stoat- weasel family and then, ahem, micturate on his rug- the one that really ties the room together, or at least did.
He does some investigation and learns that another guy, completely unlike himself but also named Lebowski, is married to a very young hot chick named Bunny who has disappeared. She owes somebody money, thus the bladder draining visit to the Dude.

Somewhere in there, the Dude's landlord invites him to a sad but moving modern dance performance.

By the way, the Dude steals a rug from the other Lebowski, to make up for the one he lost.

The Dude is a league bowler, along with some odd guys, including Jimmy Dale Gilmore, a real life pacifist who plays a pacifist bowler named Smokey. You have already heard a little about Walter, the John Goodman character. He has converted to Judaism and as they say, there is no zealot like a convert.
Walter helps the Dude with a ransom drop for the missing Bunny, who may have been kidnapped by Nihilists, including Aimee Mann, famous for being in a band called Till Tuesday in real life. There is some business about a toe.
The Dude's car gets stolen and he discovers the thief's homework ( yes, as in the dog ate mine) in the recovered car. He loses his Creedence Clearwater Revival tape in the theft.

The Dude and Walter go to visit a man in an iron lung and destroy a Chevrolet Corvette more or less because they mistake who owns it. There is a very funny "dubbing" inside joke here: when the movie was bleeped and dubbed ( i.e., bowdlerized) for TV, John Goodman said a line that was completely unrelated to the original obscene line, making the scene particularly funny for those who had seen the original.

The dude visits a pornographer and watches a video that seems to star the missing Bunny and one of the nihilists, appearing as a cable man named Karl Hungus. Eventually, he also visits Maude Lebowski, who wants him (or maybe uses him ) to extend the family line. Maude is played by Julianne Moore at the peak of her powers. There are fantasy-dream sequences.

Smokey, the Jimmy Dale Gilmore character, dies and more hilarity ensues. Several other Cohen Brothers stalwarts appear memorably, including Mr Turturro, (mentioned above) who utters a hilarious but scary line that ends "...click click click." Walter has already explained that Jesus/ Turturro is a sex offender and had to go door to door in his neighborhood telling people.

And then Sam Elliott, sipping a sarsaparilla at the bowling alley snack bar, dressed as a cowboy for no apparent good reason, and wrapping things up, utters his own trademark line about the dude, out there, takin 'er easy for all of us sinners.

And much of this is disordered, but I doubt the proper order would really help.

I am reminded of Act V scene 5 of Shakespeare's Scottish play, for some reason, maybe just to get us back on topic.