Starcraft has a fairly well-known story, but it's also a game that came out in 1998. People's memories may have gotten a little fuzzy since then. If that's you, Starcraft II's installer is here to help.
When you're installing the game, it does something few other sequels bother with, but so barely need: it provides a refresher on what has come before. Why Kerrigan looks so...angry, why Jim Raynor is on the booze, it's all explained in a handy series of title cards that play while the install meter ticks along from 0-50%.