Hmm, if I remember correctly, don't you go downstairs and free Aeris? After that, Don Corlion(sp) calls in Aeris, Tina, and Cloud (who's dressed up as a girl), and then he pulls a lever which makes you drop into the sewer.
Damn, I don't know if this is right, it sounds familiar. Try GameFAQs.com and look for a FAQ for FFVII if this doesn't help
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