Playing FFVII for the first time... question about its plot...

Okay, so far I'm loving this game. I've started it knowing next to nothing about it, which in itself is a feat given that it's touted as one of the greatest games ever...

Anyway, I recently passed the point where Cloud related Sephiroth's whole history and his own involvement with him. They climbed up the mountain to investigate this Reactor and there Sephiroth discovered his heritage. Now my question is, what were they doing up there to begin with? What was their mission? Didn't Shinra build this Reactor? And if both Cloud and Sephiroth are working for Shinra (SOLDIER), I don't think they went there to destroy it. I think I missed this plot point (as relatively minor as it probably is, though).
Just a simple reply will satisfy me. Thanks!

Jam it back in, in the dark.

Originally Posted by russ

There had been reports of monsters infesting the reactor and its surrounding areas, so SOLDIER, specifically Sephiroth and party, were dispatched to investigate and clear the area of the monsters.

That's right. Thank you.

[QUOTE=BurningRanger]For future's sake, I'd recommend you hold questions until after you beat the game.[/QUOTE
Good idea. Thanks.
Locking.

There's nowhere I can't reach.