There's a mouth in the ground (in our case, it's a hatch), and inside is some kind of prison you can't quite get to. If you've ever seen Lost, this will sound familiar... on the hatch, there's actually a series of numbers that you don't understand.
"You want to know the number to the hatch?" yells an old voice from inside. You say "yes," and the voice gives you the numbers and you're in! You go down into the prison, where there is one prisoner: Grandpa ごういち.
Note: For the ごう reading mnemonics, we use Grandpa ごういち because こういち is used for the こう reading. So just remember that ごう comes from Koichi's grandpa ごういち.