Final Fantasy VII Walkthrough
1.20: Cloud's Past
Go inside of the Shinra mansion and look around; talk to people. Look for clues about where Sephiroth is. Go into the room up the stairs, to the right, in where the blue armoured man says that he saw Sephiroth disappear to. Click on the rounded corner to find the secret entrance.
You end up in a very dark, gritty looking cave, but there is a library at the end of it; head there, then talk to Sephiroth. Leave the area. Once you have control again, head back to the library.
Follow up and out of the mansion. Head into the flaming town, where Zangan speaks to you, then go into the house with the open door.
When you have control next, you're at the reactor; head to the place where Sephiroth was showing Cloud the monsters, earlier. Check on Tifa, then go to Sephiroth.
Back to the present, Barret wants to move on. Doesn't really matter how you respond to him. Finish up your business, then head east for a small farm.