The different science fiction genres and sub-genres can be almost limitless. While all variations include elements of science and technology, sub-plots and elements related to other genres of fiction can also be added. Common science fiction genres may involve aliens, time travel, scientific discovery, as well as social changes in society. These are the most common factors included in hardcore science fiction stories, but additional components, such as romance or fantasy, may also be included. This gives writers and movie makers an almost unlimited array of possibilities for creating a sci-fi story.
All science fiction genres include some sort of future scientific discovery or technology. Even when not discussed at length in how these perceived technologies would work, they are almost always included. For instance, a story involving aliens from another planet may not go into detail on how the aliens came to earth or the technology which allowed them to do so, but it is implied that such technologies exist. Multiple mainstream science fiction genres may be combined in one work, such as a book involving both aliens and human space travel or time travel.
Science fiction is also frequently combined with other genres to create numerous sub-genre varieties. These can include almost any genre, but the most commonly implemented are fantasy, romance, and horror. Fantasy and horror are the most commonly linked genres which are considered closely related to science fiction stories. Fantasy usually involves the supernatural or make believe, such as ghosts or angels. Horror can use elements from any other fiction genre, so long as the primary focus is on scaring the reader and instilling a sense of terror.
As with most works of fiction or movies, science fiction stories are often a combination of several interwoven plots. The main, or primary, story generally involves heavy involvement with sci-fi elements of the story. Sub plots within books can include aspects of romantic fiction, literary fiction, and commercial fiction. Movies are handled in much the same way, but cinematic science fiction is usually more involved with the action of the story than with character growth and development.