The prominent action-movie maestro Michael Bay has given us the fifth movie in the Transformers toy-retail film franchise. Or maybe it is the 45th. It is difficult to tell, just as it is difficult to remember precisely how many cars were involved in a motorway pile-up in which you have been injured. We’re talking about the same steroidal infantilism as the previous four films, the same epic of tinnitus-inducing pointlessness that audiences have come to love or hate or sullenly wait to be over. Like so many of Michael Bay’s movies, it is a machine for converting your brain matter into soup, although I have a soft spot for Bay’s rather funny action-comedy Pain and Gain, from 2013.

Transformers: The Last Knight comes in at 149 minutes, and each of those minutes lasts as long as the reign of Charlemagne. Once again, normal-sized cars turn themselves into gigantic robots, while professional actors and technicians watch something comparably tumescent happen to their bank accounts. This is reputed to be Bay’s final Transformers movie as director, a farewell he clearly regards important enough to make this a colossal Return of the King-type finale. Like Tom Cruise’s recent Mummy actioner, it begins with a quaint vision of olden days Arthurian Britain, in which the Transformers’ extraterrestrial origins are made plain. This situation is created to lend instant mythic texture and purpose to what is, after all, a gigantic toy commercial. When this sense of an ending dawns on you, somewhere after the 110th minute, there is a glimmer of hope, a feeling that the Transformers ordeal might be over. But oh no. There are at least two more films in the pipeline – a Rogue One-style spin-off for the tiny Bumblebee autobot character and a sixth movie – and this future is hinted at in a final credits sting. It is cruel and usual punishment.

Optimus Prime and Bumblebee in Transformers: The Last Knight. Photograph: Bay Films/AP/Paramount Pictures

Mark Wahlberg, that dependable actorbot, is brought back to reprise the character he played in the previous film, with its infuriatingly inaccurate title of Transformers: Age of Extinction. (If only.) He is the inventor Cade Yeager, now effectively an outlaw living apart from the daughter who was supposedly important in the previous film.

Autobot leader Optimus Prime is now absent from Earth, and while other countries might have come to various accommodations with the Transformers, in the US there is conflict between them and the paramilitary Transformers Reaction Force (TRF). But this whole tense situation is prefaced by what happened 1,600 years earlier: a vision of knights of old battling on England’s greensward and Arthur’s magician Merlin being entrusted with a mighty wand by a crash-landed space creature, for the purposes of assisting Arthur in defeating the forces of wickedness, which have irradiated the good guys’ quasi-Christian cruciform breastplates. Merlin is played, gamely enough, by Stanley Tucci, who is chalking up his second Transformers appearance.

It is this staff that certain Transformers in the present day want. But the breastplate finds itself affixed, by the forces of destiny, to the ripped and buff body of Cade, who is to be humanity’s hero. Certain British academics and aristocrats are aware of the Transformers link with English legend, notably Sir Edmund Burton, played by Anthony Hopkins, a charismatic nobleman who in some occult sense is keeper of the ultimate secret. Then there is the brilliant young Oxford historian with the picturesque name of Vivian Wembley, played by Laura Haddock and, for all her academic qualifications, made to conform to Bay’s old-school views of femininity. She is, of course, to have a romantic spark with Cade.

And so the ageless, endless, pointless struggle between good and evil in the Transformer community recommences, and the final explosive showdown seems to be competing with Marvel movies for spectacle. But Marvel brings wit and fun. As far as those factors go, the Transformers franchise is in very short supply,