There’s only one thing anyone really needs to know about “Transformers: Age of Extinction,” Michael Bay’s fourth exercise in robot-on-robot violence and aggressive product placement: at nearly 3 hours, it’s the longest “Transformers” movie yet.

Any mention of the leaden dialogue, wooden acting, laugh-free jokes, deafening volume and enough explosives to knock the Earth off its axis would just be piling on.

The shame is that it didn’t have to be this way. Even though the franchise is really just one long commercial for Hasbro’s Transformers toys, the first film, had a sense of humor about itself.

The follow-up, “Revenge of the Fallen,” was unwatchable, but the third installment, “Dark of the Moon,” showed fits of inspiration amid the excess.

With “Extinction,” Bay had a chance to start from scratch. The entire human cast from previous films, including star Shia LaBeouf, is gone. Taking LaBeouf’s place at the heart of the story is workhorse Mark Wahlberg.

But the biggest reason why “Age of Extinction” should be better than it is can be summed up in one word: dinobots.

Well, the dinobots show up late in the movie and don’t have much to do but serve as handmaidens to the good-guy Autobots in their perpetual brawl with the Decepticons. What a waste.

The story this time centers on a failed, impoverished Texas inventor, Cade Yeager (Wahlberg), who with his pal Lucas (T.J. Miller), stumbles across a trashed big-rig truck. Of course, it’s not just any truck, but chief Autobot Optimus Prime.

In the previous films, humans and Autobots were allies, but now humanity is sick of all of them. But it’s up to Cade, and his (of course) beautiful daughter, Tessa (Nicola Peltz), and her (of course) brave boyfriend, Shane (Jack Reynor), to show the world the Autobots are still worth fighting for.

On the other side, the CIA, through devious Harold Attinger (Kelsey Grammer), is scheming to destroy all the remaining robots and send their parts to a robotics corporation run by the ambitious Joshua Joyce (Stanley Tucci, the best part of the movie).

Bay has been making a lot of noise about how this is the first film to use the 65 mm IMAX 3D camera and, yes, some of the special effects are impressive.

But a few more bucks on the script would have been a better investment.

Just as maddening is the heavy-handed product placement, much of it for Hong Kong and China, where some of the movie was filmed.

But there’s one part that hints at what “Age of Extinction” could have been if as much time and effort had been spent on storytelling as technology and promotion. It’s a simple chase down the exterior of a Hong Kong tenement in which Cade is running from one of the many bad guys.

Well-staged and suspenseful, the scene reaches for a “Bourne Identity” level of intensity. But this is just a brief respite from the movie’s incessant dullness.

And, with 165 minutes to fill, there’s a lot of it.