It's the dawn of the reboot-quel, a clunky new creature that resembles the movies we've loved, resurrecting characters, catchphrases and plots to lure us in. Then this imitator annihilates all previous story threads with a bit (or a bunch) of time travel so it can weave its own stories unencumbered, adding younger, sexier stars. We've seen it with "Star Trek," "X-Men: Days of Future Past" and now "Terminator Genisys," an uneven but inventive and fun entry into the sci-fi action franchise.

Directed by Alan Taylor, the film begins in 2029 Los Angeles, where a battle-worn John Connor (Jason Clarke) leads the human resistance to a major victory against Skynet, only to discover he's failed to stop the Terminator from the 1984 original from going back in time to kill his mother. So (as you probably know), John sends Kyle Reese (Jai Courtney) back to save Sarah Connor (Emilia Clarke) from certain destruction.

But when Reese arrives in the past, Sarah isn't the clueless warrior-in-the-making he's been told to expect; the timeline has been altered. She's not only well aware of her role in the war to come, but also has "Pops" (Arnold Schwarzenegger), a reprogrammed Terminator, dedicated to her protection. Together, this quirky trio sets out to take down Skynet once and for all, using time travel, a perplexing plan and more gunfire than probably any summer movie of the past decade.

Don't sweat it if you don't remember all the ins and outs of the previous four "Terminator" movies. A lengthy voiceover introduction, along with pockets of plodding exposition, will be more than enough to remind fans of the key elements. Plus, Taylor ("Thor: The Dark World," "Game of Thrones") enjoys reinterpreting the original story with a CG Schwarzenegger and some winning twists on the old iconography. But that's kind of the bummer of "Terminator Genisys": Arnold is awesome, but the movie's best bits are Schwarzenegger-centric, even though he's a supporting player here with way less screen time than his spry co-stars.

Taylor seems to realize this. His big opening battle boasts grand explosions, fleets of tenacious Terminators and scads of carnage, but what amps up this intro are the cutaways to that familiar nude-and-brawny form strutting to a time-travel platform and striking that iconic pose. Far and away my favorite part of the film was when the "old but not obsolete" Arnold goes head-to-head with the younger, naked version of himself, circa 1984. It's a clash of the titans summer movie fans will thrill to see, and the visual effects are so good it'll make your head spin. But it's a climax that comes at the beginning of the second act. While the movie finds lots of excuses to throw Pops into the fray (and we're thankful for it), no scene is as fresh and as fun as this.

Part of the problem is a little bit of history repeating. Not just that we've seen Schwarzenegger battle Terminators before, but also that "Terminator Genisys" never tires of shooting, shooting, shooting at a new Terminator, despite how little impact that has on him. I know this sounds crazy, but the barrage upon barrage upon barrage of bullets gets old. However, before this gets a bit boring, there are some real thrills to be had.

Overall, the action sequences are big, ambitious and studded with Easter eggs to "Terminator" and "Terminator 2: Judgment Day." South Korean action star Byung-hun Lee is sensational as a T-1000 who looks damn fine in uniform, and damn frightening in fighting form. (Do yourself a favor and seek out his "I Saw The Devil.") A chase scene on a school bus is exhilarating and delightfully over-the-top. And even some moments of the movie's baddie are ghoulishly entertaining. But there's something that keeps this from being a great summer movie: Jai Courtney.

"Terminator Genisys" has plot holes so big you could drive that bus through them, but that's always been true of this franchise. (Why not try to kill Sarah Connor when she was an infant?) The dizzying timelines and how exactly they plan to destroy Skynet are problems, but not ones that really bother me. But, oh, Jai Courtney.

In theory, I have nothing against the Aussie ingendude. But in film, I just don't get it. I've seen him in "A Good Day to Die Hard," "I, Frankenstein," "Unbroken," "Divergent" and "Insurgent," and none of these have suggested that he has the kind of charisma to shoulder an action movie, much less a sequel in an action franchise as adored as "Terminator." This proves it.

It's not that he's a bad actor, just a bland one. So when he's shouting at Sarah about how you can't trust machines, or reacting to twists and turns thrown Reese's way, it has little to no impact. With so many other bells and whistles, it almost doesn't matter that "Terminator Genisys"s leading man is so unspectacular. Except we're meant to believe he's the one that stone-hearted Sarah falls for, forever cementing the destiny of mankind. And even a naked time-travel tango doesn't make me believe Sarah would fall for the blandsome Reese. As an action hero, Emilia Clarke is confident and compelling, wielding weapons and delivering one-liners with equal aplomb. But Courtney is dead weight, and not even she and Pops can carry him for the film's 125 minutes.

Despite some dangling threads, a nonsensical conclusion and Courtney, "Terminator Genisys" offers enough exciting moments to make it worthwhile. Aside from Taylor's better action sequences, there's a thread of hilarious visual gags, including the comical incongruity of the crew's mug shots. J.K. Simmons brings a manic energy to a cop that's long been thought crazy for his tales of men who could become liquid metal. And "Doctor Who" fans will rejoice in a brief appearance by Matt Smith (although watching someone else time travel when he's just standing by is a little more than maddening).

But best of all, Clarke (Emilia, not Jason) and Schwarzenegger make for a wonderful odd couple, sharing an authentic intimacy, believable bravado and a sharp awareness of the material that makes "Terminator Genisys" a solid addition to the series.

"Terminator Genisys" opens Wednesday nationwide.