Poor Gordon Freeman: The bespectacled hero just can't get a break.

In "Half-Life," an experiment gone wrong opened the door to an alternate dimension of weird, unfriendly aliens. Freeman managed to travel to their world and destroy their leader, only to be given an offer he couldn't refuse: Work for the mysterious G-Man - a figure who seemed to pop up everywhere Gordon went in the disaster-stricken Black Mesa facility - or die at the hands of a horde of enraged aliens. Tough choice there.

And so begins Gordon's second-worst first day of work, in "Half-Life 2": The G-Man brings him out of some sort of stasis into a changed Earth, one firmly ground beneath the boots of the alien Combine. Gordon awakens on a tram to City 17, one of the few places where humans still live.

The place is run by Black Mesa's old administrator, the turncoat Dr. Breen, who now works with the Combine as a sort of human-alien liaison, calmly explaining from towering display screens why it's for the betterment of humanity not to reproduce, and so on.

The place is bleak, filthy and oppressive, but Freeman won't be there long before he's recognized and pursued. On the way out of town, he'll meet a few new and old friends, find a crowbar and guns, and ride a swamp boat through radioactive marshes while being pursued by a Combine gunship - and that's before things pick up.

Of course, Gordon will have to fight his way through all this. He'll contend with apparently human Combine troops, whose creepy breath masks and bullfrog voices hint at their alien origins. He'll fight off several varieties of headcrab, nasty little critters that can take over the bodies of hapless humans. And he'll shoot down that gunship eventually - and this is all in the first few levels. There are many more foes, human and not, to meet and defeat.

Gordon makes use of many of the weapons of the first game, including that trusty crowbar, a magnum revolver, a machine pistol and other conventional weaponry, as well as more exotic devices. But the best of all is the new gravity gun.

With this gadget, Gordon can attract and repel objects with ease, allowing him to clear passages, create routes and fling explosive barrels or saw blades at hordes of headcrab zombies. It's great fun, and an ingenious way to make the most of the game's excellent physics.

As good as the action and game play are, the visuals might be even better.

"Half-Life 2" gives "Doom 3" a run for its money in the detail department, and wins hands-down in terms of design. Characters and creatures are incredibly realistic. The backgrounds are expansive and just as detailed, and all this coupled with the physics engine makes for an eerily realistic sci-fi setting. The sound is also terrific. From weapon and environmental sound effects to the screeches and growls of alien wildlife, everything sounds distinct - and often unnerving.

The graphics and sounds come together with the story to create a great sense of mood. The player does everything from Gordon's perspective, so there's nothing to break the illusion of living Gordon's experiences, and the story is told obtusely, through sketchy conversations and inference. It's not even clear how long Gordon has been removed from the world, though he appears not to have aged in the meantime.

The game comes in two boxed retail versions from Vivendi Universal Games and in three downloadable packages available through the game's developer at www.steampowered.com. All versions of the game automatically communicate with the Steam service when played; one must have an Internet connection to play this game in any way.

The different packages include a number of extras: The basic boxed version comes with "Half-Life 2" and an upgraded version of the mega-popular "Counterstrike" online shooter. The top-notch Steam edition comes with those things, plus a revamped version of the original "Half-Life" and a hefty load of swag.

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Half-Life 2

4 stars

SYSTEM: PC

PUBLISHER: VU Games/Valve

HOW MUCH: $49.99-$89.95

AGE RATING: Mature