The best science fiction plots are meant to stretch the mind for possibilities and to make the reader aware that alternatives to perceived reality may exist in the world. Science fiction plots also seek to explore the possibility of the existence of other worlds and life on other planets. Such plots may include: an outbreak of a fatal disease or virus, time travel, parallel or alternative universes, and the existence of other dimensions.
Classic plots may portray a main character or characters who become possessed by an alien form in mind or body. Space exploration or colonization is another common science fiction element. Plots dealing with fighting, or saving, an alien civilization are also popular science fiction characteristics. Time warps where the day of the characters keeps looping over and over and the heroes must find a way of stopping it is another recognizable science fiction plot point.
Science fiction characters are most often divided into categories of species, and are as numerous and varied as the genre itself. Science fiction characters can include human, alien, monster, or some hybrid or mutation of all three. They can also include artificial or computer intelligence as well as robots and cyborgs who reflect or resemble humans.
Science fiction in literature has long been used as a platform for social criticism, although some fiction critics believe science fiction as social commentary is a genre all its own. Science fiction as social commentary seeks to critique the nature of humanity, its faith, values, dependence on technology, and the future of the planet.