There is no way to describe the arc of the program without making it sound like an incidental soap opera of our short and ordinary days. Tony becomes a cabby, fails as a publican, confesses his marital infidelity on camera, buys a home in London’s outskirts, adds a vacation property in Spain, raises his granddaughter when his own daughter proves unable, tries and fails to open a sports pub, sells his home in Spain after the financial crisis and through it all talks about the celebrities he has driven in his taxi. John, one of the posh boys, drops out of the show before “28” and only returns at “35” on the condition that he can draw attention to the charity work he does in Bulgaria; he never makes it to Parliament, to his chagrin, but he wears the wig and the silk as a Queen’s Counsel. He never has children. (John, who declares in “35 Up” that the show feels like a “little pill of poison” injected into his veins, will not speak to Apted on camera; Lewis has conducted the interviews since 1991.) Bruce, who once dreamed of missionary work, teaches immigrant children in the East End — it’s the same school Tony attended, as it turns out, after the neighborhood’s great demographic shift — before marrying late, having two boys and abandoning the ideals of his youth to serve at an elite school that was founded in the year 948. They gain lined faces, put on weight, lose parents.

The original political aims of the series weren’t abandoned so much as rendered implicit. Apted experimented with various timely inquiries over the years — he once asked the participants about the death of Princess Diana — but the material never worked and he threw it out. Class, of course, never goes away. Though there is indeed some social mobility, the elites are running the country and the nonelites, while mostly comfortable, are not. In 2005, with the release of “49 Up,” the British journalist Jonathan Freedland wrote, “It seems Granada’s original premise — that background determines fate — has held up depressingly well.” The nature of the class system, however, had changed since the Thatcherite revolution of the early 1980s. As the British social historian Joe Moran noted, in 2002, the series “did not foresee the decline of the British economy’s manufacturing base, the fragmentation of the working class, the rising number of white-collar jobs and Thatcherism’s destruction of union power.” It also didn’t foresee the expansion of middle-class consumerism or the rise of the predatory gig economy.

Apted might not have anticipated these things, but they nevertheless find expression as the delimiting conditions of the participants’ lives. If in the early installments he attempted to recruit individual biography to dramatize socioeconomic history, the program’s attention is ultimately drawn to an even more profound dynamic: the interplay of self and environment. The narrative center of gravity of the “Up” films hovers somewhere between the stiff-necked documentarian and the unruly subjects to whom he is yoked. Apted, like a social scientist, emphasizes the role of big, obstinate forces; his participants almost invariably take the opposing side of agency and self-determination. What we get, as the show goes on, is an ever-fuller picture of how particular individuals at times shrink to inhabit the givens of an inheritance and at times spill over the sides of those constraints. What emerges are the countervailing qualities of structure and dignity.

The program is able to generate this surfeit of meaning in part through the frictional trajectory of its participants’ relationship with Apted himself — as both an individual and as a sort of imago, a figure of fraught authority. Apted has been candid about the odd, transactional nature of the exercise. The children retain an enormous amount of power over him. Many of them have made no secret of the fact that they wish they’d never been chosen; they endure it only out of loyalty. Which is not to say that they aren’t happy to exploit Apted’s vulnerability. Peter, who dropped out after the right-wing media tarred him for the vehement anti-Thatcherite politics he expressed in “28 Up,” was only coaxed to return when Apted agreed to help him promote his band. Still, the negotiations could be extremely difficult and often drove Apted to the end of his tether. He spent several “ludicrous” months trying to get Suzy to participate in “63 Up” — in which, among other ruses, he used other people’s phones to call so she wouldn’t know it was him — and she nevertheless bowed out, to his enduring peevishness.

When they have agreed to continue, Apted nevertheless has needed to remain cautious; he gets right up to the line of the unacceptable without crossing it. His perennial gambits, as he once acknowledged to an interviewer, are “Why?” and “What do you mean?” He recognizes that “why” is an aggressive question, yet he is perfectly happy to ask it and then sit in silence, to the point of sadism. “I never want to take advantage of them or be too soft on them,” he told me. “With people like Neil, who are very bright but very vulnerable, you don’t want them to think you’re being too judgmental.” Still, he can be frontal to an astonishing degree. At the end of his interview with Neil in “63 Up,” Apted risks asking what Neil makes of the maxim, which in Neil’s case would have predicted something other than a life of free-fall.