The story takes the form of a long letter written by one old university friend to another. The recipient of the letter, known only as "Thos" (a college nickname, short for "Doubting Thomas") introduces the strange document, from a friend known only as "Cass" (another nickname; short for "Cassandra", an allusion to the friend's apparently prophetic abilities). Cass is regarded as a harmless eccentric by his friends, but Thos notes that his prophecies and preoccupations, wild as they may seem, have a habit of coming true. Thos also notes, ominously, that Cass's letter "bears the address of a well-known mental home".