After completing the Lighten Up mission in Assassin's Creed Odyssey: Judgment of Atlantis, you will get a quest called The Birds and the Bees.
This mission will begin in the Garden of Triton. You'll meet a woman there. You have to catch her up.
At some point, a woman will disappear from your eyes. You need to move the stone board placed on the left. Thus, you will open a secret passage. Move to another room where you'll find a woman.
When you start talking to her, Neokles will appear and provide you with some information.
The mission will end when you put the stone in a special place and return to the building.