As with other story modes in games like the recent Mortal Kombat, "A Shadow Falls" basically consists of standard, one-round Street Fighter V matches bookended by lengthy cinematic scenes to tell the story and explain why those fights are happening in the first place. So if you're familiar with SFV gameplay, you'll feel right at home. I was a little disappointed not to see more innovation or new gameplay styles here, but on the other hand, that's probably not what fans of the series are looking for. It's more about letting you play some matches with your favorite characters while getting more backstory than any other SF game has offered thus far.
Again, though, how much you get out of that story will depend on how well you know the Street Fighter universe. As someone who only knows the basics (M. Bison is bad, obviously), I felt a bit lost in the game's mythology as the story started. That said, a lot of the mystery comes from the way the presentation unfolds. By the end of the first chapter, I had a decent sense of what was going on, though the motivations for many of the characters still weren't clear. That'll likely become more obvious as the game goes on, but those with less SF experience should expect to be in the dark at first.
The gameplay, however, will be familiar to anyone who has played a recent Street Fighter game. The match-ups are identical to other battles in the game, except for the fact that they only last one round (rather than best of two out of three). What does make the fighting interesting is that from fight to fight, you don't know the identify of the character you'll be controlling. A few times, I found myself in control of one of the "good" guys for several battles only to have the viewpoint switch up as I needed to dispatch the character I was previously in control of. Although that formula is standard in story mode for fighting games, it was still fun to see how your next match would get set up.
This update will also contain a few other bits of new content beyond story mode. Most notable is a new character named Ibuki. She's showed up in other Street Fighter games, but her style and moves have been revamped for SFV. Probably the most notable thing about her gameplay style is that she has to "reload" her projectile weapons. She holds five knives at a time and can fire them all off at once or in various combos -- but once she's out, you'll need to find a quick break in the action to initiate a reload.
You'll also be able to purchase in-game items using cold, hard cash rather than earning in-game "fight money" as you play. Capcom also says there will be a handful of new stages and costumes and teased a few surprises that I didn't get to see just yet. The exact launch date hasn't been finalized, but Capcom promises the mode will be available by the end of June.