1) Ryu meets his childhood friend, Ken.
2) Ryu picks a fight with some badass villain who is plotting to take over the world. Examples; Sagat, M. Bison/Vega, Akuma, and that neon guy from Street Fighter 3.
3) (optional) Chun Li steps in with all her panty-revealing high-kick glory.
4) A bunch of other characters step in to either join Ryu, or the bad guys.
5) Ryu beats the crap out of all the said characters.
6) Ryu beats the crap out of Ken.
7) Ryu beats the crap out of the bad guys.
8) Ryu goes out to pick a fight with some other bad guy, thus starting yet another Street Fighter upgrade/sequel.
So basically, if you've played one game, you've played them all.