Batman: The Dark Knight Returns, Pt. 1 was a DC Universe Animated Original Movie set to be released on September 25, 2012. It is an adaptation of the Batman story arc, The Dark Knight Returns by Frank Miller and a sequel to the 2011 animated film Batman: Year One, which was also a Frank Miller story.
Because the story was split into two films in order to be told in its entirety, Batman: The Dark Knight Returns Part 1 and Part 2 are a fairly close depiction of the Frank Miller story The Dark Knight Returns.
At this point, it is known that Frank Miller's main Batman comic book stories all take place in the same universe. Known in the DC 52 Multiverse as Earth-31, the works included in this universe are Batman: Year One, The Dark Knight Returns, All-Star Batman and Robin, the Boy Wonder, and The Dark Knight Strikes Again. Because of this and fact that the Dark Knight Returns films are thought of as sequels to the Batman: Year One film, one can assume they should also be placed in the same universe.