A storybook that says: "Once upon a time..."
"Once upon a time" was a literary phrase used to introduce a narrative of past events, typically in fairy tales and folk tales, and in retellings of myths, fables, and folklore in general, as well as in children's books or other children's stories. These stories were sometimes accompanied by a happy ending.
The Kingdom of Elysian was one story that began with this phrase.
In 2259, USS Enterprise Chief Medical Officer Joseph M'Benga used the phrase before explaining what happened to him in the five hours that the rest of the crew didn't remember experiencing. According to M'Benga, all good stories began with "once upon a time". (SNW: "The Elysian Kingdom")
The holographic program Flotter and the Tree Monster began with the phrase. (VOY: "Once Upon a Time")