"Sredni Vashtar" is a short story written by Saki (Hector Hugh Munro) between 1900 and 1911 and initially published in his book The Chronicles of Clovis. It has been adapted for opera, film, radio and television.
The story concerns a sickly ten-year-old boy named Conradin, who lives with his strict cousin and guardian, Mrs De Ropp. Conradin rebels against her and invents a new religion, idolising as a vengeful, merciless god a polecat-ferret he names Sredni Vashtar. Conradin keeps the ferret hidden in a cage in the garden shed, and worships it in secret. The story comes to a climax when his cousin discovers his god.
Conradin, a sickly 10-year old boy, lives in the care of his despised, overbearing and controlling cousin Mrs De Ropp. He relies on his vivid imagination not only to keep him strong enough to survive, but also to serve as his escape from the real world. Rebelling against Mrs De Ropp's oppressive care, Conradin secretly cares for two animals in an unused garden shed: a hen, which he adores, and a polecat-ferret, which he fears and keeps locked in a hutch. Gradually, Conradin begins to venerate the ferret as a god, naming it Sredni Vashtar. He worships it weekly, bringing offerings of flowers and berries, and stolen nutmeg for special occasions.
Mrs De Ropp grows concerned over Conradin's visits to the shed. She discovers the hen, and sells it. She announces the sale to Conradin, expecting a protest, but to her surprise the boy does not respond. But in secret, he changes his worshipping rituals and asks of his god an unnamed boon: "Do one thing for me, Sredni Vashtar".
When Conradin's visits to the shed do not cease, Mrs De Ropp investigates further, and discovers the locked hutch. Suspecting guinea pigs, she ransacks his room, finds the key, and goes down to the shed, forbidding Conradin to leave the house. While she is gone, Conradin slowly begins to accept defeat, knowing that his god was not real and that his cousin will come out of the shed in triumph. But when Mrs De Ropp fails to reappear after some time, Conradin begins to chant a song of victory. Eventually he sees the ferret emerge from the shed, with dark wet stains around its jaws and throat. It passes out into the garden.
A sour-faced maid announces tea and asks for Mrs De Ropp. Conradin tells her that she has gone to the shed, and makes himself a piece of buttered toast. As he enjoys his toast, there are screams from the maid, calls for help from the kitchen staff, and later the sound of something heavy being dragged into the house. As voices discuss fervently who should break the news to the boy, Conradin calmly makes himself another piece of toast.
On September 15, 1941, an adaptation of "Sredni Vashtar" began the premier episode of the CBS Radio series The Orson Welles Show. Blanche Yurka portrayed Mrs. De Ropp, with Conrad Binyon as Conradin and Brenda Forbes as Matilda.
"Sredni Vashtar" has been adapted as a chamber opera three times. In 1988 the composer Robert Steadman and the author Richard Adams wrote the 75-minute Sredni Vashtar. In 1996 Cuban-born composer Jorge Martin and librettist Andrew Joffe with the American Chamber Orchestra produced Beast and Superbeast, a group of four chamber operas based on stories by Saki, including "Sredni Vashtar". Martin also composed a Piano Fantasy on Sredni Vashtar  In 2010 the story was again adapted by Nicholas Pavkovic and Jim Coughenour and performed at the San Francisco Conservatory of Music.
This story was adapted for American television and aired on a ghost anthology series called Great Ghost Tales on August 24, 1961. "Sredni Vashtar" was the basis of the 1979 horror film The Orphan, also known as Friday the 13th: The Orphan, by the director John Ballard. In 1981, the short film Sredni Vashtar by British director Andrew Birkin won a BAFTA award and was nominated for an Oscar. In 2003 Angela M. Murray produced a version of the story in the Tartan Shorts series for the BBC, set in Scotland and including shadow puppetry. "Sredni Vashtar" was further adapted with two other Saki stories for a 2007 broadcast on BBC4 titled Who Killed Mrs De Ropp?
The story also inspired film directors of the Czech Republic three times. Vaclav Bedrich made a cartoon film based on it in 1980. Martin Faltyn's 1981 graduation film from the Gerasimov Institute of Cinematography is an adaptation of "Sredni Vashtar". In 1995, Pavel Marek adapted the story into his graduation film from FAMU.
"Sredni Vashtar" was adapted as a single narrative song for the musical Saki Shorts by John Gould and Dominic McChesney. The one serious item in the show, it stays faithful to the story with the addition of a twist in the last line that hints it is being sung by (the adult) Conradin himself.
The Seattle punk band Steel Tigers of Death also has a song titled "Sredni Vashtar." The lyrics reference the story, including the chorus, "Sredni Vashtar, do one thing for me! Sredni Vashtar, kill!"
Jean Rhys references the tale in the eponymous story of her 1976 collection of short stories, Sleep It Off Lady, in which the protagonist, Miss Verney, feels terrorized by a large rat hiding in her garden shed, at one point calling out aloud, "Come out, come out, Shredni [sic] Vashtar, the beautiful.".