Spoilers incoming for Justice League, so if you haven't seen it yet you've been warned.
As you know, Superman was killed fighting Doomsday at the end of Batman v Superman: Dawn of Justice and the beginning of Justice League finds a world left in mourning at the loss. That includes Clark's friends and family, but also Bruce Wayne and Diana, and it's the former's idea that ushers in the character's return.
Cyborg contains Mother Box technology within him, and that allows him to interface with it directly. That leads to Batman's brilliant idea of using the tech to bring Clark back to the world of the living, something Wonder Woman and Aquaman seem to protest. Batman's plan wins out in the end, so they bring the body back to the Kryptonian ship and give it a jolt from the Mother Box. Thanks to Flash and Cyborg, the plan works...sort of, and Superman flies out of the ship.
He lands by his memorial, and the League follows. Something is off though, and he doesn't seem to be all there. After an accidental defense mechanism from Cyborg, Superman attacks the team, with Wonder Woman attempting to slow him down. An amazing battle ensues, culminating in a new showdown between Batman and Superman. This one isn't even close to a fight though, but Batman has a secret weapon....Lois Lane.
Lane's presence triggers something in Superman's mind, and the two fly off together.
The Justice League has to initiate a full assault on Steppenwolf's base of operations, and together they manage to infiltrate it and find the three Mother Boxes. Unfortunately, Steppenwolf is getting the upper hand thanks to his huge army of Parademons, but a familiar voice is heard just in the nick of time, and Superman appears to fight by the League's side.
Superman also helps Cyborg defuse the Mother Boxes, and ultimately ends up back in the world of the living. Superman is back, and his return is glorious.
You can judge it for yourself, as Justice League is hitting theaters now.