The first part of the chapter is overshadowed by a serious worry. Portly, the Otters' little boy, is missing, and has been missing, as Rat explains, for several days. The Otters have looked all over for him and asked everyone about him, but he is still gone.
Mole asks why be concerned, as Portly is always wandering off, but Rat explains that the length of time he has been gone makes this a much more worrying situation. The fear is that he might have drowned.
Mole and Rat go looking for Portly, and come to a enchantingly lovely island, led there by beautiful music. They go ashore, and both are overwhelmed as they come face to face with a vision of the Divine—a profound experience of God himself:
"This is the place of my song-dream, the place the music played to me," whispered the Rat, as if in a trance. "Here, in this holy place, here if anywhere, surely we shall find Him!"
Then suddenly the Mole felt a great Awe fall upon him, an awe that turned his muscles to water, bowed his head, and rooted his feet to the ground. It was no panic terror—indeed he felt wonderfully at peace and happy—but it was an awe that smote and held him and, without seeing, he knew it could only mean that some august Presence was very, very near.
They do find Portly on this island, but as dawn comes, the vision of God disappears. The title of the chapter therefore refers to the Divine force—God is the "piper" of the chapter's title. He is at the "gates of dawn" because Mole and Rat have their experience of him just before dawn breaks, in a liminal period between night and day. Portly has not wanted to leave the island because, although he cannot put words around it, he experiences there the comforting and beautiful presence of God.
The Otter's boy is missing. Portly, has been lost and Rat informs Mole of the missing child. They set out on the river at night to go look for the child of Mr. and Mrs. Otter. As dawn is breaking Mole begins to hear beautiful music from a pipe. Rat does not hear it at first, but as they go farther down river and the dawn becomes lighter Rat also hears the beautiful music. The two friends realize that they don't hear anything but the music.
"..the two animals moored their boat at the flowery edge of the island....the piping had stopped... There in the clearing stood the Friend and Helper. They could see the backward sweep of the curved horns, gleaming in the growing daylight. ...the long graceful hadn still holding the pan pipes. Last of all, nestling between His very hooves, sleeping soundly, Mole saw the little, round, pudgy, childish form of the baby otter."
The mythical creature had protected the baby and summoned the two friends with his musical pipe at dawn. This is the meaning of the title "The Piper At The Gates Of Dawn."