The story's protagonist is a boy named Conradin who is slowly dying of an unspecified disease. Conradin lives with his older cousin, a woman named Mrs. de Ropp whom he despises. Unknown to Mrs. de Ropp, Conradin has a pet ferret which he keeps in a tool shed in the garden. Conradin names the ferret Sredni Vashtar and begins to develop his own religion in which Sredni Vashtar is a god. When Mrs. de Ropp decides that she does not want Conradin to go to the tool shed anymore, Conradin repeatedly prays to Sredni Vashtar, asking the animal deity to do just one thing for him.
There have been numerous adaptations of "Sredni Vashtar" to other media.
Conradin is a ten-year old boy who has been told that he has no more than five years to live. He lives with his cousin and guardian Mrs. de Ropp. Although she would not even admit it to herself, Mrs. de Ropp dislikes Conradin. The boy hates Mrs. de Ropp. He is able to hide his true feelings from her but takes a lot of pleasure from doing thins which he knows will irritate her.
It is Conradin's good imagination which has prevented him from falling into despair. He dislikes the garden in which he is allowed to play because he can be easily observed by Mrs. de Ropp from one of the house's many windows. He finds refuge in a tool shed. Conradin pretends that the shed is inhabited by many of his imaginary friends. It is also home to two real animals, a Houdan hen which Conradin loves and a ferret in a hutch which is kept hidden in semi-darkness. Both the ferret and the hutch were secretly sold to Conradin by a young butcher's assistant. Conradin loves the ferret but is also afraid of the sharp-toothed creature. After some time, he gives it the name Sredni Vashtar.
In reaction against the church services which Mrs. de Ropp makes him attend on Sundays, Conradin starts to develop his own religion in which Sredni Vashtar is a god. Thursday is the sacred day in Conradin's religion. On that day, he makes offerings of red flowers or red berries to the ferret. Conradin also sometimes holds religious holidays in celebration of various events, such as Mrs. de Ropp's bad toothache. On those days, he presents Sredni Vashtar with offerings of stolen nutmeg.
Mrs. de Ropp decides that she does not want Conradin to go to the tool shed anymore. She announces one morning that the Houdan hen has been sold. In an attempt to console the boy, Mrs. de Ropp allows Conradin to eat toast, a treat which is usually denied to him, that afternoon. Conradin returns to the tool shed that evening and says a prayer to the ferret which he has never said before. He says, "Do one thing for me, Sredni Vashtar." Conradin repeats the same prayer in the tool shed each evening and in his bedroom each night.
After Conradin continues to go to the tool shed, even after the hen has been sold, Mrs. de Ropp decides to investigate it. She finds the hutch. She asks Conradin what is inside it but he does not answer. Mrs. de Ropp searches Conradin's bedroom, finds the key to the hutch and returns to the tool shed. Conradin watches from the dining room window. He continues to pray to Sredni Vashtar but knows that it is just pretend and his ferret will soon be taken away from him. Out of sadness at the prospect of soon losing the animal, Conradin invents a new hymn in which Sredni Vashtar the Beautiful brings death to his enemies.
Conradin notices that Mrs. de Ropp has not come back from the shed for some time. He begins to feel a sense of triumph and quietly sings his hymn again. He sees the ferret, which has blood stains around its mouth, come out of the shed. It drinks from a small stream in the garden before going through a hedge and disappearing forever.
A maid tells Conradin that his meal is ready and asks where Mrs. de Ropp is. Conradin answers that she went to the tool shed some time ago and did not return. Conradin listens to the panic which ensues when the servants find the dead body of Mrs. de Ropp in the tool shed while calmly making two slices of toast for himself.
Film versions of "Sredni Vashtar" include Sredni Vashtar by Saki (USA 1940), The Orphan (USA 1979), Sredni Vashtar (UK 1981), Sredni Vashtar (Mexico 1987) and Sredni Vashtar (Czech Republic 1995). The 1981 movie Sredni Vashtar, directed by Andrew Birkin and starring Sacha Puttman as Conradin and Judy Campbell as his guardian, won the BAFTA Award for Best Short Film.
"Sredni Vashtar" has been adapted for television as episodes of the American series Danger (first shown on CBS on January 26, 1954) and Great Ghost Tales (first shown on NBC on August 4, 1961). An episode of the British short film series Tartan Shorts is based on the story. It stars thirteen-year old Fergus Nimmo as Conradin and Sian Thomas as his guardian. It was first shown on BBC Scotland on November 27, 2003. The British TV movie Who Killed Mrs. De Ropp? is based on three short stories by Saki, "Sredni Vashtar", "The Story-Teller" and "The Lumber Room". It features Gemma Jones as Mrs. De Ropp and Samuel Honywood as Conradin. It was first shown on BBC Four on May 2, 2007.
An abridged version of "Sredni Vashtar" is read by Jonathan Pryce in the seventh episode of the British children's TV series Spine Chillers, first shown on BBC 1 on November 27, 1980. Tom Baker, best known for playing the Fourth Doctor in Doctor Who, read "Sredni Vashtar" for the British TV series Late Night Story. "Sredni Vashtar" was intended to be the first episode of the series and was intended to be broadcast on BBC 2 on December 22, 1978. Due to industrial action, the episode did not air that evening. It was finally shown on British television for the first time on September 24, 2007.
A chamber opera based on "Sredni Vashtar", with music by Robert Steadman and libretto by Richard Adams (best known as the author of Watership Down), was first performed in 1980. Another chamber opera based on the story, by Nicholas Pavkovic and Jim Coughenour, was first performed at the San Francisco Conservatory of Music in 2010. A section based on "Sredni Vashtar" is included in Beast and Superbeast a 1996 chamber opera with music by Jorge Martín and libretto by Andrew Joffe which was inspired by the works of Saki. A song about the story is included in the 2003 musical Saki Shorts by John Gould and Dominic McChesney.
Songs about "Sredni Vashtar" are included on the 2002 album Drawing on Other People's Heads by the British absurdist electronic band Wevie Stonder and on the 2003 album The Burning Season by the American gothic rock group Faith and the Muse.