secret house underwater

You are a person who was swimming underwater and suddenly met a person who lived underwater. He saw you and swam up to you:
Hey, you're very handsome, take this stone, if you're interested, lean this stone against that wall and I'll swim over and open the door, let's sit there, don't be shy, swim over!
You stood there like a pillar, not knowing what to do, but accepted the stone as it floated to the surface.
You were still in school and swam underwater during breaks.
When the next change came, you decided to dive underwater to find out what would happen if you applied the stone that the guy gave you.
You applied the stone, and the guy floated over a few seconds later and then opened the door, inviting you in. His house, the Cave, was cozy and spacious, but cold for you.