'...Grown-ups don’t look like grown-ups on the inside either. Outside, they’re big and thoughtless and they always know what they’re doing. Inside, they look just like they always have. Like they did when they were your age. The truth is, there aren’t any grown-ups. Not one, in the whole wide world.’ [loc. 1580]The Ocean at the end of the Lane is much darker, on reflection, than its surface suggests. Our nameless first-person narrator, returning to the place where he grew up, recalls his childhood, and his friendship with Lettie Hempstock and her 'mother' and 'grandmother'. (It's probable that the relationships here are rather more complex than the narrator's interpretation of them.) The Hempstock women mention their previous home ('the old country') in passing, but the narrator isn't able to form a coherent impression. The reader, though, might end up suspecting that they're mcuh, much older than they seem. Maiden, mother, crone? These aren't moon goddesses -- vice versa, if anything.
The plot? A hole in Forever is torn: something, uninvited, comes through: it must be banished. There is always a price to pay.
There is much more to it than that, of course. Gaiman's writing is so emotionally authentic that it's easy to slip into reading it as autobiography. ("I make art, sometimes I make true art, and sometimes it fills the empty places in my life. Some of them." [loc. 91]) This is a layered novel, the child-narrator's memories overlaid by the adult-narrator's sense of loss. Much has been forgotten -- but the realisation that he has forgotten a great deal already, more than once, is no comfort.
Part of what makes this chilling, melancholy story so effective is Gaiman's evocation of the joy and powerlessness of childhood. Children have no agency: things happen to them. Children can take pleasure in small, ephemeral things while the world around them crumbles. Children think that one day they'll be all grown up and confident and different. Adults -- at least the adults who still remember what it was really like to be a child -- know that isn't true.
"How can you be happy in this world? You have a hole in your heart. You have a gateway inside you to lands beyond the world you know. They will call you, as you grow. There can never be a time when you forget them, when you are not, in your heart, questing after something you cannot have, something you cannot even properly imagine, the lack of which will spoil your sleep and your day and your life, until you close your eyes for the final time..." [loc. 1948]