The Star Trek television episode (from either the 'Next Generation' or the 'Voyager' series) that most stands out in my mind is one where the starship meets up with a celestial whale.

In the episode, a guest character makes good friends with one of the regulars, but is suffering from the noise of thousands of voices in his head. By the end, he leaves the relative security and community of the starship for the dispassionate connection with the whale, and the peace and silence he experiences traveling within its dream-like being. His new friend, though, is left behind. The visual image of the whale sailing away from the ship into the deep of the galaxy is etched in my memory.