Some of the more famous stories in Unknown were the novels Lest Darkness Fall and Divide and Rule by L. Sprague de Camp, Sinister Barrier by Eric Frank Russell, The Incomplete Enchanter and the other Harold Shea stories by de Camp and Fletcher Pratt, Darker than you Think by Jack Williamson, Conjure Wife by Fritz Leiber, and the short stories "Trouble with Water" by Horace Gold, "It" by Theodore Sturgeon, and "Smoke Ghost" and the early Fafhrd and the Gray Mouser stories by Leiber. Unknown also published the first science fiction or fantasy stories by Leiber ("Two Sought Adventure"), Sturgeon ("A God in a Garden"), Anthony Boucher ("Snulbug"), and James H. Schmitz ("Greenface").
In keeping with its relation to Astounding Science Fiction, the style and approach of the stories in Unknown was a hybrid between those of fantasy and science fiction. The premises of many of the stories were caused by unexpected, and often humorous, situations in which a traditional fantasy creature or concept is introduced into an otherwise ordinary world. For example, in de Campís "Nothing in the Rules" the manager of a swimming team tries to win a race by including a mermaid on the team.
Other stories take the opposite trope of applying a scientific logic in a traditional fantasy setting; the Harold Shea series is based upon the premise that traditional fantasy worlds, such as those in Norse mythology, are based upon the rules of mathematical logic. (See rivets and sorcery.)
Unknown also included some science fiction which used the themes of traditional fantasy. For example, Darker than You Think uses the premise that werewolves exist, but that they are not supernatural, and explains the traditional beliefs about them without using fantasy.