Penglai appears in Jackie as a place the immortals talk about the way grandparents talk about a hometown they left — affectionately, infrequently, with a small pause before the name. In the Chapter 6 dining-hall scene, Lan Caihe mentions in passing that the apricot tree on the basket-table at the Council was clipped from a Penglai stock; Zhang Guolao corrects her, drily, that the original is older than the comparison. Lü Dongbin tells Jackie that the peaches on the lacquer dish in front of him are not Penglai peaches — "those are not for thirteen-year-olds yet" — and Jackie, who had not been thinking about peaches, suddenly is. The book uses the isles to set the scale of what immortality actually means without ever transporting Jackie there.
Penglai is also where the Council's deepest archive is said to be kept, in a passing line from Cao Guojiu in Chapter 19 — the records of every prior incarnation of Nezha live on a shelf in a room on an island that may or may not be reachable from the world Jackie is in. The line is given without elaboration. Jackie's structural choice is the same one it makes with the Celestial Palace: the most important off-page locations are kept off-page on purpose. Penglai is real in the book exactly because the book never goes there.
Mount Penglai first appears in the Shanhaijing (Classic of Mountains and Seas, c. 4th c. BCE) and is canonized across the Han, Tang, and Song dynasties as the principal Daoist immortal-isle, alongside Fangzhang and Yingzhou. In 219 BCE, the First Emperor of Qin dispatched the alchemist Xu Fu with a fleet and three thousand young men and women to find Penglai and return with the elixir of immortality; Xu Fu sailed east and never came back. The story has been told as both failure and sly success — the standard reading in late-imperial poetry is that Xu Fu reached the isles and chose not to return.
The peach-of-immortality tradition attaches firmly to Penglai through the figure of Xi Wangmu, the Queen Mother of the West, whose orchards bloom there and whose peach-banquets convene the upper ranks of xian on the thirty-thousand-year cycle. Sun Wukong's theft of these peaches — and his subsequent flight — is the comic-tragic engine of the early Journey to the West.
The peaches grow here. Penglai is the cosmographic source of the immortality fruit — the orchard where Xi Wangmu's peaches ripen on a thirty-thousand-year cycle.
The hometown the immortals left. The Eight Immortals and other xianren retain ties to Penglai but operate from below — the isles are origin and reservoir, not residence.
The unreachable archive. Cao Guojiu's line about Nezha's prior-incarnation records placing them on a Penglai shelf turns the isles into a literary off-stage memory bank.
Off-page on purpose. Jackie never sails east. Penglai's power in the book is the same as the Celestial Palace's — kept luminous by being kept unvisited.