The teaser starts with the opening lyrics of "Come As You Are" and a wall of various articles and images that talk about Daredevil, Jessica Jones, Luke Cage, and Iron Fist. Pieces are torn away and individual letters are revealed. D. E. F. E.
As the imagery is fully stripped away, we see another image of a Hand grasping these letters, now spelling DEFEND, and the voice of Stick saying "You think the four of you can save New York? You can't even save yourselves."
We then get our first look at the actual logo for The Defenders. The logo reveals a darker tone than the Disney/Marvel Cinematic Universe that the Marvel Universe on Netflix is famous for.
Based solely on this teaser, we can assume the main plot will involve our heroes defending New York against The Hand, the ninja clan seen in seasons one and two of Daredevil. What leads these four to join forces and become a team of Super Heroes? Will we see cameos from other "street level" heroes?