Part 1: I fucking told you soI fucking told you so
Shepard's in jail because we killed a couple (hundred thousand) Batarians. But trouble's afoot. Something big is coming. Of course, we know what that something is. Reapers are invading and Earth is fucked. We run into Kaiden who now outranks us. James is a good guy, but his role is to fill new players in on the state of things so he's not super exciting. He's way better than soldiering than Ashley though, who is terrible in 3. Her hair is dumb, her outfit is dumb and her powers don't work right so I'm glad we left her to die on Virmire. Well, that and "being a goddamn racist" didn't help either.
Many improvements have been made to gameplay overall. Shepard is way more mobile. You can jump gaps, mantle up onto things, dodge roll, and now can infinitely sprint. Combat is fast, frenetic, and full of explosions. Especially for our new and improved Vanguard.
The morons who stuck us in jail get blown up and Anderson and Shepard have to make it to the Normandy to get the fuck out of Dodge. Husks used to be shambling zombies, now they are very fast zombies. They also sit on your face if you let them get too close so don't let that happen. You'll still take damage while you're dislodging them and can die, especially on Insanity. We run into the most hated child ever in videogames. If you haven't played the game you'll see why later. Anderson thinks we should go to the Citadel and ask the Council for help. Y'know, since they've been so helpful in the past. I'm sure that won't be a wasted trip.
After some forced resource depletion we get our ship back and Hackett orders us to go to Mars where our girlfriend has been doing research. There we're supposed to find something that will help us defeat the Reapers once and for all. I'm sure this will all go off without a hitch.