Back in 2006, Pixar Studios was on something of a hot streak. They were doing some of their best work, both financially (“Finding Nemo”) and creatively (“The Incredibles”). And then they released “Cars,” which still made boatloads of money, but was easily the weakest of the Pixar repertoire. Now, after four years of producing not only some of the best animated films of their respective years, but some of the best films period, Pixar returns to sequel-making with “Cars 2,” which is not surprisingly its weakest film since its forebearer.

The first film was a simple love story between Lightning McQueen (Owen Wilson) and the small town charms of America. Its sequel couldn’t be a bigger departure, casting McQueen and the rusty, dented tow truck Tow Mater (Larry the Cable Guy) overseas for an international racing competition.

And then the film strays into the absolutely ridiculous by introducing secret agent Finn McMissile (Michael Caine), who, by a farcical series of misunderstandings, ends up working with Mater to save all the cars participating in the race (including McQueen) from a nefarious plot.

While the film’s spy plot provides most of the entertainment found in “Cars 2,” it also makes some colossal mistakes. The biggest is placing Mater in the center of the film and sidelining Lightning McQueen except for the occasional race scene. Mater only occasionally worked in the first film, and that’s when Larry the Cable Guy reigned with his shtick. But placing him front and center couldn’t be a worse choice creatively. As a character, Mater is nothing short of grating, and watching him stumble his way through various scenarios ripped off of the Bond and Bourne films alike only underlines how irritating Larry the Cable Guy’s delivery is without Owen Wilson.

Another problem with this story is how it requires its characters to be absolute idiots. First off, Mater is completely oblivious to the fact that he’s involved in espionage. Even worse, his spy colleagues didn’t immediately recognize that Mater couldn’t be further from a secret agent, which is a bit harder to swallow.

However, as ridiculous as the spy plot may be, it adds quite a bit to the film. It’s always interesting to see how a Pixar film handles real life-or-death stakes, and from its thrilling opening scene, it’s clear that “Cars 2” will have a body count. While none of its main characters are ever in too much danger, much of the film’s spy bits work mostly because Pixar doesn’t shy away from the ugly consequences of gunplay (as ugly as they can be in a G-rated film about talking cars anyway).

As always, even when they’ve stumbled creatively, Pixar has made an absolutely gorgeous film. Their rendition of Tokyo (punnily renamed Towkyo) is a marvel, and the film’s action is uniformly exciting. “Cars 2” shines in its all-too-brief racing scenes, which are a rush of color on their own but are given real weight and depth by the 3-D effects, which are among the best of the summer.

Also good is the customary pre-film short, which is presumably the first in a series of “Toy Story Toons,” lovely little postcards that allow us to catch up with Woody and Buzz. Unfortunately, even this lacks the entertaining simplicity that comes with some of Pixar’s other shorts, but is still sweet in a low-key way.

What makes the “Cars” franchise such a misstep is that it’s everything Pixar isn’t. For the last decade, Pixar has been making films that tell heartfelt, human stories set in a world with some semblance of internal logic, and the “Cars” films couldn’t be less logical or less human. The biggest questions looming over both films, one that is entirely glossed over, is why do cars exist in a world without humans? Who built them? And more importantly, why do they have interiors? When the film’s world doesn’t make a lick of sense, it’s hard to get invested.

The nonsensical nature of the world might actually work if it was a bit easier to invest in these characters. The “Toy Story” films are built around a similarly ridiculous premise but take place in a world that is recognizably ours, and more importantly, have characters with hearts and souls, something the “Cars” films sorely lack.

So we’re left with the second film in a franchise that has never really worked for Pixar, one built around celebrities voicing talking objects and lots of merchandising opportunities for cars with silly names. While kids will most likely love the colorful, never boring “Cars 2,” fans looking for the maturity and heart they’ve come to expect from Pixar will be sorely disappointed.