The DC Expanded Universe continues to grow in its attempt to compete with Marvel's Cinematic Universe, and the franchise is about to unleash its own incarnation of Avengers with Justice League. The new film will feature Wonder Woman, Batman, Superman, Aquaman, The Flash, and Cyborg teaming up to save the world from the alien invader Steppenwolf and his army of Parademons. That may seem like a big leap for just the fifth film in the DCEU franchise, but the seeds for the film were actually planted in 2016's Batman v Superman. So what happened in Batman v Superman that paved the way for Justice League?
Batman v Superman was the second film in the DCEU following 2013's Man of Steel. That film introduced the character of Superman, and BvS introduced a ton of other characters who will go on to feature in Justice League. First and foremost was Batman, obviously, but the film also introduced Wonder Woman (who of course went on to have her own prequel origin film) along with The Flash, Aquaman, and Cyborg. The movie also dropped a few hints, if you were really paying attention, about the alien invasion that's coming in Justice League. So just how did it set up all of this? Gather 'round and prepare to geek out.
The movie shows how the arrival of Superman has affected everyone in the world, especially in the immediate vicinity of Metropolis and its twin city, Gotham. In this world, Batman has been fighting crime for many years, and he is weary of this all-powerful alien who is suddenly calling the shots. In short, he doesn't trust Superman because he's too powerful. Superman, for his part, isn't too crazy about Batman either and thinks that he's too aggressive in his vigilantism, believing that he should leave the crime-fighting to someone more, well, like him.
Bruce Wayne meets Diana Prince, and the two of them discover that Lex Luthor is researching more people with abilities like Superman — metahumans. In addition to Diana, the other metahumans Luthor is researching are The Flash, Aquaman, and Cyborg — all of whom are briefly seen on video within Luthor's files. Batman's fears about Superman are exacerbated after he has a dream of a dystopia filled with Parademons in which Superman is king. He is awoken from this dream by The Flash, seemingly traveling from the future, who cryptically warns him that Lois Lane is "the key" and to "find us." It's a super weird scene that makes zero sense within the context of the film, but is clearly setting up something in Justice League.
The tensions between Batman and Superman eventually come to a head and the two fight, with Batman coming out on top thanks to his use of kryptonite. However, he refrains from killing Superman when he finds out that both of their mothers are named Martha. The two immediately become best friends and unite to take down Luthor, who creates a monster using Kryptonian DNA named Doomsday in order to kill Superman. In the ensuing fight against Doomsday, Diana Prince joins the fray, revealing herself to be Wonder Woman. The three heroes are losing the fight until Superman attacks Doomsday with a kryptonite spear, and the two of them kill each other.
After Superman's funeral, Bruce Wayne tells Diana that he plans on putting together a team of superheroes — including her — as a sort of tribute to Superman, and to protect the world from alien threats since Supes is no longer around to do so. At the very end of the film, some soil on Superman's casket floats, hinting that he may not actually be dead.
That's Batman v Superman in a nutshell, and it essentially lays the foundation for Justice League by 1) a vision of the film's alien invasion, 2) killing Superman, which inspires Batman to form the Justice League, and 3) hinting that Superman is not really dead. These are the three main plot points of Justice League — Batman puts together a team to stop an alien invasion and Superman returns to help — and they were all originally laid out in BvS. So if you plan on watching Justice League, you should probably check out BvS first if you haven't seen it.