SPOILER WARNING: This article contains SPOILERS for Justice League, out today.
Batman v. Superman: Dawn of Justice may have ended with the death of Superman, but the movie itself closed with a faint ray of hope in the form of the dirt rising from his grave. As an audience, and this being a film based on comic books, we all knew that there was simply no way that Superman could be gone for good. After all, the next big movie in the DC Extended Universe of movies was already confirmed to be the superhero team-up extravaganza, Justice League. Surely, such a movie wouldn't happen without Big Blue as one of the team's members. He is, after all, the team's leader, their main source of inspiration and easily their biggest powerhouse.
Therefore, it was no big secret that Superman would be a part of the film, and it certainly isn't a spoiler for anyone. While the trailers, posters and banners for the film never showed him, it was perhaps the least well-kept secret that Superman was the sixth member of the Justice League, given that Henry Cavill himself never pretended otherwise. While it was nice of the marketing department to play the mystery card and actually allow Superman's return to be a genuine surprise in the film, the true mystery would always be how exactly he came back to life. Well, now that we have seen the film, we have the answer.
Just like in the comic books, Superman had died following a devastating and brutal battle with Doomsday. But while Clark Kent was brought back to life thanks to a Kryptonian regeneration armor in the source material, the movie's return played out much more differently. It was a bit more complex and ingrained in the story, and it directly involved the rest of the Justice League members. Bruce Wayne may spend the better part of the first third of the movie trying to recruit Barry Allen, Arthur Curry and Victor Stone to his cause, but he very well understands the kind of power they are going to face, and that none of them are powerful enough to stop what's coming. He knows that the one thing they neede, that the entire world needs, is Superman.
When Steppenwolf and his Parademon forces come into the possession of two out of the three Mother Boxes they need to unleash their hellish plan on Earth, Cyborg brings the final Box to his new teammates. Before long, Bruce theorizes that the alien technology that they now possess is an energy source the likes of which they have never seen before. He believes that, in the stasis chamber of the Kryptonian ship located in Metropolis -- the same ship and chamber where Lex Luthor used General Zod's body to create Doomsday -- a Mother Box discharge could reanimate the dormant cells of Kal-El and that this could, theoretically, be used to return Superman to life.
Various members of the League are quick to protest, like Diana and Arthur who believe that there is no way of knowing that what they would bring back would truly be him. Diana warns that this could very well lead to another Doomsday situation, and that perhaps it would be better to let the Kryptonian rest. Barry Allen names the famous Stephen King novel Pet Sematary as an example of how things could turn very badly. But Victor Stone, who understands the Mother Boxes better than all of them combined, thinks that Batman is right, and that this plan could work.
Barry and Victor dig up Superman's grave, and finally the six Leaguers enter the Kryptonian ship in Metropolis with the Mother Box and Clark Kent's body. No one is exactly certain that what they are about to do will work, but they go through with it because they know the situation is dire, and because they have hope. Aquaman lays Clark's body in the Kryptonian waters of the ship, but a complication presents itself when Cyborg realizes that the ship's power is too low, and that there isn't enough energy to get the process going. As the lightning-powered Flash, Barry is quick to say that the electrical discharge from his powers might just do the trick. All he needs is to cover enough distance, and to activate the Mother Box at the exact same time as it touches the water.