"Terminator Salvation" shares several similarities with 2007's "Live Free or Die Hard." Both are the fourth entries in their respective aging franchises. Both are rated PG-13—a first for the more organically R-rated subject matter. And both, it turns out, are more successful at mounting impressive, large-scale action sequences than they are at providing dramatic support and fully fleshed-out characters. Fortunately, the pared-down MPAA rating does not show signs of compromise; while blood and gore are minimal, the tone is gritty and the violence is just as brutal as it has always been. In the annals of the series, "Terminator Salvation" is more on par with 2003's "Terminator 3: Rise of the Machines" than 1991's groundbreaking "T2: Judgment Day," but director McG proves, just as he did with 2000's "Charlie's Angels" and 2003's "Charlie's Angels: Full Throttle," that he knows his way around a camera.

At Longview State Correctional Facility, circa 2003, death row inmate Marcus Wright (Sam Worthington) signs away his body to the cancer-stricken Dr. Serena Kogan (Helena Bonham Carter) and the company she works for, Cyberdyne Systems, just before he receives his lethal injection. Fifteen years later, he reappears to discover a world that has been destroyed by nuclear warfare. The handful of survivors, spread out across the U.S., are in a fight for their lives against the reigning, state-of-the-art killing machines built by San Francisco-based Skynet. As Marcus struggles to figure out where he has been for over a decade, he runs into teenage warrior Kyle Reese (Anton Yelchin) and mute youngster Star (Jadagrace) and subsequently teams up with them. When word comes over the radio that the people of the resistance are gathering together to take on Skynet headquarters, the three of them set out to find the transmitter: one John Connor (Christian Bale), unknowingly Kyle's future son.

In an old audio recording explaining to John the importance of finding Kyle and keeping him alive to ensure his own ultimate existence, his late mother (Linda Hamilton, in a vocal cameo) remarks, "A person could go crazy thinking about this." Indeed, "Terminator Salvation" is convoluted, but, with the exception of a change in actors from movie to movie, it segues rather seamlessly from the past films and comes close to reaching full circle with the welcome, albeit sneakily digitized, third-act appearance of Arnold Schwarzenegger. The emotional element takes a backseat to the special effects and pyrotechnics in the screenplay by John Brancato and Michael Ferris (2007's "Primeval")—usually a harbinger of doom—but the pacing is so ceaseless and the technical credits boasting such showmanship that one hardly notices as they watch.

Unlike Michael Bay, who has no idea how to build an action scene so that it is both comprehensible and exciting, director McG is markedly more adept. The cinematography and editing are fluid, aiding in tension and not resorting to flash cutting or an overuse of shaky-cam, and the visual effects are superb. Whereas the CGI in the recent "X-Men Origins: Wolverine" looked shoddy, filled with obvious green screen and seemingly unrendered money shots, the effects here actually look like time, effort and care was put into every last detail. The results help to depict a plausible post-apocalyptic world (shots of an obliterated San Francisco, with a rusted Golden Gate Bridge crumbling but still standing, are appropriately eerie).

The central stars this time are Christian Bale (2008's "The Dark Knight"), a gruff, humorless flatliner as John Connor, and virtual unknown Sam Worthington (2002's "Hart's War"), stealing the show from everyone else as Marcus Wright. Bale takes part in some snazzy action set-pieces—an elaborate one near the beginning that portrays a helicopter crash and a surprise attack by a terminator in only two shots is close to mind-blowing—but otherwise has little to do. It doesn't help that, in taking over the role from Edward Furlong and Nick Stahl, he has turned John into an outright two-dimensional bore. Worthington, however, sizzles with the one impressively written character, a man who yearns to be virtuous while not understanding the truth behind what he has become. The actor jumps in and out of his natural Australian accent at times, but otherwise demonstrates terrific charisma and an intriguing character arc.

The supporting cast is hit and miss. Though not the first person one might consider for the part of a tough, gun-toting hero, Anton Yelchin (2009's "Star Trek") adds gravel to his voice and disappears into his part as Kyle Reese. A chase sequence pitting himself and Marcus against invading terminators is one of the adrenaline-fueled highlights, leading to a breathless run-in on a narrow bridge. As Kyle's sidekick Star, young newcomer Jadagrace is saddled with the most pointless character, an unctuous ploy to have a kid involved in the goings-on. Lending uneven femme support, Moon Bloodgood (2009's "Street Fighter: The Legend of Chun-Li") is fetching as Blair Williams, who takes a liking to Marcus when he saves her from danger, but Bryce Dallas Howard (2007's "Spider-Man 3") is criminally underused as John's pregnant wife, Kate. What drew Howard to a part that calls for her to stand around in dank underground lairs and look solemn is anyone's guess, but it does the actress no favors.

"Terminator Salvation" ends on a dopey note that screams of being a post-production tack-on. It's really quite lazy, and fails to push the story further. What isn't lazy are the almost two hours that come before it. By being set after the bombs have gone off, the film is enlivened with fresh aesthetic surroundings and a grim, urgent fortitude. The various incarnations of the murderous terminators show originality and a very present danger, and the climax set at Skynet headquarters exhibits hints of 1986's "Aliens" (that's a good thing). "Terminator Salvation" does not give the viewer a whole lot to munch on after the fact, and the characters, for the most part, remain ciphers. Still, the film is pulpy and fun as a rousing summer diversion, and more than respectable for being the fourth part in a twenty-five-year-old series. There's still some life in it yet.