This was supposed to be the Season of Science Fiction on the major broadcast networks.

It hasn't quite worked out that way.

CBS aired nine episodes of "Threshold" before pulling the plug, leaving four unaired hours sitting in its storage vault.

NBC was so pleased with "Surface" that it upped its original order of 13 episodes to 22. Then it cut that order back to 15 and decided to call that a complete season — complete with a finale that resolved nothing.

The only one of the three aliens-invade shows that stands a chance of finishing its first season and returning for a second is ABC's "Invasion," and that's not a sure thing. The show airs its first episode in more than a month tonight (9 p.m., Ch. 4), and the plot thickens. More.

If you missed the first 17 episodes, there are these alien things in the water that have a nasty habit of grabbing humans, sticking big pokey things into them and then turning themselves into replicas of that human. They come complete with the human's memories and feelings, so much so that not all the "hybrids" know they've been, well, alien-ated.

We don't know exactly what all this means, but we do know sheriff Tom (William Fichtner) and his wife, Mariel (Kari Matchett), are hybrids. And that Mariel's first husband, park ranger Russell (Eddie Cibrian), knows as much about what's going on as anyone who isn't one of the hybrids.

Tonight, Russell and his brother-in-law, Dave (Tyler Labine), find out about the hybrid army on the secret island; Russell's current wife, Larkin (Lisa Sheridan), has a run-in with a guy she's sure is a hybrid; and Russell and Mariel's teenage son, Jesse (Evan Peters), has a run-in with hybrids at high school.

Yes, it's sort of weird and convoluted. But the story actually progresses — unlike the show that precedes "Invasion" on ABC, a little thing called "Lost."

My greatest fear, however, is that "Invasion" will be yet another show that ends without telling us what the heck is going on. And that's growing increasingly tiresome.

AT LEAST "ALIAS" FANS won't be left hanging — we're promised a genuine wrap-up to that series, which returns tonight (after a four-month hiatus) with back-to-back episodes (7 and 8 p.m., Ch. 4).

There's a lot going on tonight — the birth of Sydney's (Jennifer Garner) baby; the return of Michael Vaughn (Michael Vartan), who's supposed to be dead; and the return of Irina Derevko (Lena Olin).

There are four episodes after tonight in a somewhat-abbreviated 15-episode fifth season, and the finale will be a genuine finale.

And, quite frankly, 103 episodes is a lot for a show that was never a real hit in the ratings. It gathered a cult following that followed Syd and the gang through a series of wild twists and turns but never broke out — even last season, when it aired after the white-hot "Lost."

GHOST AND DEMONS have been more successful than aliens on network TV this season. CBS has already renewed "The Ghost Whisperer" for next season (although that show is about as much in the science-fiction genre as "Touched by an Angel" was).

The WB's "Supernatural" turned out to be the best of the science fiction/fantasy shows of the season — it's genuinely scary and a lot of fun to watch. It hasn't been renewed for next season yet but stands a good chance of making the cut when the WB and UPN combine into the CW.

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E-mail: pierce@desnews.com