Aliens have a high profile in today's television and movie science fiction. But even as humans are invaded by, conspired against, and just plain eaten by aliens on the screen, SF novelists are wondering, "What if there's nobody out there after all?"

The question may stem from a new appreciation of the difficulties of interstellar travel, or perhaps it's just a fashionable reluctance to create yet another alien species.

It's the end of the 29th century, and man has colonized nine worlds and explored thousands more. Not one of them has shown the slightest trace of life.

Most of humanity isn't too concerned. Life on the Nine Worlds is comfortable -- energy and food are plentiful, and there's work if you want it or a guaranteed living allowance if you don't.

Human society is changing, though. It's not decaying -- the Nine Worlds' prosperity is equitable and sustainable -- but it is turning inward.

One by one, the starliners and exploration ships that journeyed between the Worlds are being put in mothballs. After a millennium of strange new worlds, it seems like there's nothing left to see.

Despite a few dedicated souls, astronomy is rapidly becoming a hobby. The starship Hunter -- the last expedition to look for intelligent life among the stars -- came home a generation ago, and it looks like there will never be another.

Enter Dr. Kim Brandywine. An astronomer by training, Brandywine is a fundraiser for the Seabright Institute, one of the last organizations still watching the skies for aliens. She's also the clone-sister of a member of the Hunter expedition.

Brandywine's elder sister disappeared hours after the Hunter returned to port. For years, Kim has accepted her sister's disappearance as an unsolvable mystery -- until the uncle of another expedition member contacts her about a cryptic message.

All the members of the expedition have died since the Hunter's return, but Kim finds herself asking: what if they found something after all?

Much of what follows could almost be mistaken for a conventional mystery novel, a fact McDevitt acknowledges with frequent references to "Veronica King," the most popular fictional mystery character of the Nine Worlds. Brandywine travels around the colony of Greenway, piecing together clues about the last days of the Hunter expedition.

It's an entertaining story, but the suspense doesn't really kick into gear until it becomes obvious that yes, the crew of the Hunter really did find somebody out there -- and that the aliens they encountered may not be friendly.

It's not just that McDevitt orchestrates a couple of chilling encounters with the aliens midway through the book. Their presence raises the stakes for the characters, turning what might have been a depressing novel about giving up into a challenge for humanity.

The difference between the first and second halves of Infinity Beach illustrates a fundamental difference between traditional mysteries and SF writing. Where mysteries are largely self-contained -- somebody in the cast knows what's really happening, even if it's not the hero -- SF characters frequently find themselves trying to understand a complete unknown.

While most SF characters and their fans are excited by the unknown, McDevitt makes a point of showing that not everyone feels the same way. Even after the aliens are encountered in Infinity Beach, several characters present plausible reasons why getting to know them may not be such a good idea.

In the end, Infinity Beach comes down firmly in favor of going out to meet the aliens. But as in the novel, whether we keep looking for them in the real world may depend on the fine balance between those who fear the unknown and those who embrace it.