Like Nesbit's Railway Children, the story begins when a group of children move from London to the countryside of Kent. While playing in a gravel pit soon after the move, they uncover a rather grumpy, ugly and occasionally malevolent sand-fairy known as the Psammead who is compelled to grant one wish of theirs per day.
The children's wishes are:
The effects of the wishes generally end at sundown.
In Five Children and It, the Psammead is described as having “eyes [that] were on long horns like a snail’s eyes, and it could move it in and out like telescopes; it had ears like bat’s ears, and its tubby body was shaped like a spider’s and covered with thick soft fur; its legs and arms were furry too, and it had hands and feet like a monkey’s” and whiskers like a rat. When it grants wishes it stretches out its eyes, holds its breath and swells alarmingly.
The Origin of the Psammead: The five children find the Psammead in the gravel-pit, which used to be seashore. There were once many Psammeads but the others died because they got cold and wet. It is the only one of its kind left. It is thousands of years old and remembers Pterodactyls and other ancient creatures. When the Psammeads were around, they granted any wishes, mostly food for the families. The wishes would turn into stone at sunset if they were not used that day. (Chapter 1)
The name Psammead, (pronounced “Sammyadd” by the children in the story) appears to be an inventive Greek pun coined by Nesbit (from the Greek ψάμμος "sand" after the pattern of dryad, naiad, oread, etc.) upon the name of “Samyaza” the leader of the Grigori (“Watchers”, from Greek egrḗgoroi) supernatural creatures of antediluvian myth. Knowing the pun's in-joke, shows the logic at work behind the creature's phobia of water “nasty wet bubbling sea”, and why the eyes are placed watchfully upon the end of long horns like a snail's eyes.
"They did see it [the Psammead] again, of course, but not in this story. And it was not in a sand-pit either, but in a very, very, very different place. It was in a— But I must say no more."
Some fifty years later, the premise of Five Children and It inspired the plot of Half Magic (1954) by the American author of children's books Edward Eager.
The Return of the Psammead by Helen Cresswell (1992) concerns another family of Edwardian children who visit the White House and discover the Psammead.
In 1991 the BBC turned the story into a six-part series. In the UK it was released under the story's original title; in the USA it was released as The Sand Fairy. This was followed by The Return of the Psammead in 1993, with the Psammead the only character linking the two series. Both series were scripted by Helen Cresswell.
A NHK/Tokyo Movie Shinsha production named Onegai! Samia Don was broadcast from 2 April 1985 to 4 February 1986 with a total of 78 episodes produced. An English version was never produced, but it came out in other languages, for example, being known in French as "Sablotin."