REBIRTH opens with Sephiroth floating in darkness, defeated after Advent Children. He's reflecting now, and he is beginning to realise all the mistakes he made. Somehow his empathy has awakened amidst the Hades lifestream he is in, and all he can think is to try and make amends. He visualises the planet before him, and then shoots toward it like Superman. We hear Cher's If I Could Turn Back Time in the background, and Sephiroth through sheer willpower manages to shoot through a time portal into the past. As he lands in a Midgar alley, crackling with electricity as if in Terminator 2, he sees Aerith and then quickly hides behind a wall.
We see Aerith glance around and then hurry off as she does in P1.
"I'll make things right...I'll change it all!" Sephiroth swears to himself. The logo appears on the screen as we watch him walk down the alley. FF7 REBIRTH.
During the game we find out that Sephiroth wanted the party to defeat the whispers in p1 because they were Time Travel Police from the lifestream trying to stop him changing the future, so Sephiroth manipulated Cloud and co into beating them with him. After this he tries to tell Cloud the truth, that he is only there to help and change the future, but he is very bad with making his good intentions understood, and thus Cloud refuses and attacks him. Sephiroth blocks and dodges everything then peacefully leaves, just as we saw in p1. Seph ensures that Zack survives this time by defeating the second army that was supposed to come after him.
Seph then thinks his job is done, but he finds out that Genesis intends to complete what happened in the OG because it is more poetic and fits Loveless. Genesis defeats Seph in their one on one battle because of his new Planet Powers that Minerva gave him, so Sephiroth begs Cloud and co to join their team. He sobs and hits the floor as he confesses to his past crimes.
Aerith: But now you've changed that future...you've redeemed yourself, Sephiroth! You even saved Zack!
Zack: Thanks bro.
Eventually Seph hugs them all and they decide they will team up to defeat Genesis. Game ends on this note as they set off to confront Genesis at the North Crater...