Jetstream is a pretty simple DLC story prequel to the events of Metal Gear Rising: Revengeance. In here, you play as the titular Jetstream Sam before joining Desperado and becoming one of the baddies in the main game, and it's a surprisingly fun time at that.
Based mostly on chapter R-04 of the main story, it's an hour and something of going around slashing enemies as a familiar but tankier (and thus a tad bit more complicated) character that lacks the augmented abilities Raiden has in his cyborg form, and save for all of its rehashed locations and assets it does have its fair share of animated cutscenes and unique enough boss fight to be a sweet little appetizer after the fact.

But that's pretty much it, I don't think it'd absolutely be that worth it to buy on its own so I'm glad it's included into the PC port of the game.