Only Marty will be playable and, as an adventure game, most of the gameplay will revolve around collecting and using objects (which you can see a glimpse of in the corner up above). The game also has an extensive dialog system, and the story is a full sequel to the three movies. It starts out "set several months after the events of the third movie from Marty's perspective," but of course with time travel, the game could revisit any of the previous three movies or head off to anywhere else. Heavy.
There will be some familiar locations -- you can see the 1955 billboard from the end of the second movie in another screenshot below, and you'll visit both Doc's lab and the Hill Valley town center (with clock tower, of course) as well. And you'll "see them in ways you haven't experienced before," promises Telltale.