The basic idea of this story is that a 10 year old boy named Conradin (who is terminally ill) is living with his cousin, Mrs. DeRopp. They hate each other. She is pretty harsh with him, always trying to make him unhappy.
Conradin has a pet hen that he loves and a pet ferret (they are both in a shed out in the garden) that he comes to worship as a god. He prays to it, among other things, to have bad things happen to Mrs. DeRopp.
Mrs. DeRopp eventually finds out about the pets and decides to get rid of them. When she does Sredni Vashtar the ferret kills her (seemingly in response to Conradin's prayers).
The story ends with Conradin happy because this has happened.