Utnapishtim tells Gilgamesh the story of the flood and his own unrepeatable survival of it, making clear from the outset that his immortality was a singular exception granted by the gods rather than a status Gilgamesh could hope to earn or acquire through effort. To underline the point, Utnapishtim challenges Gilgamesh to a much smaller test first, simply to stay awake for six days and seven nights, and Gilgamesh, exhausted from his long journey, falls asleep almost immediately, failing even this modest trial and demonstrating how far he remains from any claim on true immortality.
Taking pity on him as he prepares to leave, Utnapishtim's wife persuades her husband to tell Gilgamesh of a thorny plant growing on the seabed capable of restoring youth, which Gilgamesh dives for and retrieves, planning to test it on an old man back in Uruk before using it himself. On the journey home, while he bathes in a pool, a serpent snatches the plant and eats it, immediately shedding its old skin, an episode the epic offers as an explanation for why snakes renew themselves while humans do not. Gilgamesh returns to Uruk without the immortality he sought, but the poem closes with him instead taking pride in the great walls of his city, a legacy that will outlast him in a way no potion could have provided.