Fairy tales happen in the long ago.
Author Setterfield sets the tale along the banks of a fictional River Thames in an unspecified time that feels a hundred years or so before ours.

Fairy tales have a problem that needs to be solved.
On the evening of a winter solstice in the 19th century, a drenched man bursts into the Swan Inn with what appears to be a large doll. The doll turns out to be the corpse of a little girl, and then not a corpse at all. Who is this child?

Fairy Tales have fantasy and make believe in them.
Is it possible magic is at work at the Swan, or is there some unknown scientific reason the child, who hours before appeared dead, was very much alive? Once Upon a River is a story that straddles the line between reality and fantasy, between science and the supernatural, between the land of the living and the dead.

Fairy Tales have clearly defined good vs evil characters. 
There are shadowy figures galore--tramps and fortune-tellers, wastrels and unpredictable madams, goodhearted parsons and psychics who may or may not be real psychics. 

Royalty or the well-to-do are usually present in a fairy tale.
Is the mystery child the kidnapped daughter of a well-off local couple who was taken two years ago? 

There may be giants, elves, talking animals, witches or fairies.
One shadowy recurring figure, the subject of much of the fireside storytelling at the Swan, is Quietly, a ghostly ferryman who gave up both life and death to save his own daughter. He sees to it that those who get into trouble on the river make it safely home again. 

Fairy tales have happy endings with a lesson or a theme.
Diane Setterfield's novel is about the mysterious space between life and death, exploring the ways in which stories not only keep us hopeful, but keep us alive.