''Star Trek'' and ''X-Files'' fans show their devotion online by churning out their own stories about the characters

Pity the hapless network executive, hard at work on the fall-TV lineup, trying to forget last season’s decline in viewership — a whopping 4.25 percent.

In cyberland, though, prime time is alive, well, and spinning out wild plots that will never run on Nick at Nite. Chris Carter may bar X-Files Mulder and Scully forever from consummating their deep bond — but it’s a common fantasy in the archives of fan fiction, where familiar TV and movie characters populate digital poems, scripts, short stories — even full-length novels — that span hundreds of websites and Usenet discussion groups. Readership figures are elusive, but fanfic’s America Online index page has collected a half million hits in the past year.

Fanfic’s roots lie deep in the ‘zinedom of the ’70s, when Star Trek devotees cranked out mimeographed manifestos on galactic doings. Web publishing — more sophisticated and much more powerful — has nurtured a blooming of imaginative fanfic that deviates significantly from prescribed story lines. One subgenre, ”crossover,” posits a universe in which characters from different shows (and networks) exist in a single hyperactive universe; one story weaver has Law & Order detectives Logan and Briscoe working a murder case with the X-Files duo. Another variety, ”slash,” creates sexual histories more appropriate to the Kinsey than the Nielson report.

Though networks and movie studios themselves are no strangers to recycling, particularly when it comes to vintage TV shows, Hollywood’s scribbling class tends to look down on the Web-work amateurs. Anyone can write an episode using Seinfeld characters, they say; the actual invention of a new story line and new characters is the hard part.

Not that Seinfeld inflames the fannish heart. Nor do most of TV’s other top 20 shows. While The X-Files has inspired about 130 fanfic sites, the more highly rated ER and Friends tally 6 and 3 respectively Seinfeld, none that Alta Vista knows about. Star Trek and its lineal descendents — The Next Generation, Deep Space Nine, and Voyager — are the Nancy Drews of electronic chronicles, with more than 165 sites to date. Obviously, cyberscripters respond to science fiction motifs, and their inactivity is turned on by intense, ambiguous intimacy between characters whose emotional makeup is otherwise poles apart. TV may provide end users with better-quality video and sound, but only computers let the audience play with the mix — and each other.