Brambleclaw has a dream where he is drowning in salt water and then sees a cave with teeth. Ravenpaw says that it is a real place and he knows other rogues and loners who had been there. Ravenpaw says it looks like the sun is being swallowed by the sea each evening, hence the name.
After several cats have prophetic dreams from StarClan, telling they have to meet Midnight and listen to what she has to tell them, they set off on a journey to the sun-drown-place. During the journey, the cats have dreams or illusions of tasting salt water. When they arrive, Brambleclaw falls into the water, but is rescued by Stormfur and Squirrelpaw. They find the cave where they meet Midnight.
When the Tribe and Clan cats journey to the mountains, they meet Purdy, and Squirrelflight explains to the apprentices that they met him on their first journey, and he helped them find the way to the sun-drown-place.