Head to the Temple in Shanoah. Try to talk: nobody's there, but you can head inside. Do so. Talk alternately to the standing man and the lying man until a third character shows up. When everything's finished, leave this room: you'll be back at the Temple menu. Select "Talk" again. The scenario begins.
Once you enter South Creek, you won't be able to leave again without forfeiting the scenario. This includes losing battles: if your party is annihilated, you'll wake up back in Shanoah, and you won't be able to return to South Creek. So make sure you've got some healing items, and save at the inn before you leave! It's probably preferable to make sure everyone's at level 5, too, if you can stand it - but you can always level up when you get there. When you're ready, exit the town and select "??."
Cross the bridge. There'll be a short scene with Hybard. If you head east, you'll come to a ruin. Examine the rightmost block to find an inscription. To enter the house, examine the door twice. Head right and down, and examine the book on the table a few times. Then go upstairs and enter first the room on the left, then the one on the right. If Hybard doesn't appear, go back downstairs and examine the book again, then come back. After a long conversation you'll get your bodies back.
Follow the good doctor to the ruins. He's not there, so head back to the house and examine the window three times. You'll jump up and get the book from the table. Examine the window again. Now return to the ruins and read that inscription. It's rather cryptic, so you need to think about it a bit: leave the screen, then enter again. Presto: intrat deus ex machina! So go ahead, knock at the Gate of Darkness three times. An entrance will open, which you can enter.
At the moment you can go back to the save point by the bridge to rest if necessary - save any potions you may have for later. When you reach the upper floor there's no way down again, so level up now if you think you need to.
Head west. Read the notice and examine the machinery, then go through the doorway. Head up to the interesting apparatus in the top left. Examine the source of the light three times, then the left-hand window four times, and finally the left-hand pedestal twice. Now the lift is working, so go back out to it and ride it up.
Examine the crate in the top right corner of the room three times, then leave the room by the door to the south. Note that the Undead Wolf monsters you meet in this next area are exceedingly nasty. Definitely run unless you really know what you're doing.
You can't open the middle door, so go through the right-hand one. Examine the pile of sacks twice, then the table in the middle of the room. Go up to the place from which the rat appeared and examine the wall. Now examine the desk to your left, then push it under the hole you saw. Climb onto it (the drawer serves as a foothold) and examine the wall twice, then go back to the left-hand room and take the wedge from the crate. Return to the desk and climb back onto it, and examine the wall twice more. Get back down and take the candlestick from the table to the right, then back up and you'll be able to climb up to the hole. Head up and left. There are two gratings. Look through the right-hand one, then head over towards the other. When you regain control, take four steps. You'll wake up back on the bridge again.
The scenario will now end when you leave South Creek. If you leave right away you'll receive 1600 exp; if you go and listen at the front door first you'll get 1800. Now when you return to the Sealed Cave you'll see that one of the crystals of the seal at the entrance is lit up.