Deep down, Michael Kirk, the veteran investigative reporter and on-air correspondent behind some of PBS Frontline's most trenchant and impactful exposés, always suspected his fifth time co-producing The Choice would feature a last-minute twist unlike any he had seen before.

On the one hand, PBS's once-every-four-year biography of the two candidates competing in an upcoming presidential election is relatively straightforward: These are the candidates, this is who they are, these are the influences that shaped them. Now you decide.

The Choice is intended to be objective. In a freewheeling conference call with TV reviewers last month, Kirk insisted his primary aim is to keep his own opinions to himself. PBS is a public broadcaster, and by giving the two candidates equal time — and equal scrutiny — The Choice is performing a public service.

Kirk is no fool, though. He is not naive. And while he admits that, even with his experience and advanced years on the job — The Choice 2020: Trump vs. Biden (Tuesday at 9 p.m. ET, check local listings) marks 20 years since he first started poring over presidential candidates' pasts for clues as to who they are today — he always knew this one would be different. All presidential elections are meaningful, but some are more meaningful than others.

And with everything that's going on — controversy over immigration policy; the ongoing culture wars; simmering tensions over the Black Lives Matter movement and the attendant street protests; a growing climate emergency as evidenced by deadly wildfires on the U.S. West Coast and an unprecedented hurricane season on the U.S. East Coast — it seems as if America, both the nation and the idea, are at a crossroads.

The Supreme Court was always going to be a battleground. Whoever wins on Nov. 3 could conceivably appoint two or even three justices to the court bench, potentially shaping social policy for an entire generation to come. The stakes are frighteningly high.

That said, Kirk is at pains to point out that The Choice is biography, not news. That shields the program to some extent from the blow of a last-minute surprise like the sudden passing of Justice Ginsberg, one of a minority of liberal voices on the court bench.

At its heart, The Choice asks fundamental questions about who the candidates are, and why their lives have turned out as they have, Kirk said.

"Is personality destiny?  Do the times make the person, or does the person make the times?  Those two fundamental questions are the things I wrestle with every time we approach any film, but especially these big presidential biographies."

This time, whether by design or by chance, The Choice has tapped a rich vein, where straight biography is concerned.

"They are about the same age," Kirk said. "So they've (lived) the same American historical chronology. We can measure them against some of that, Vietnam, the Civil Rights movement, all those events."

Despite that, Kirk noted, "Their lives have been sort of weirdly contradictory. Trump has been so out front and on-camera, so much in the daily news, in the newspapers, tabloids, he's been on The Apprentice.

"Meanwhile, Biden is the longest-serving public servant who has ever run for president. He's been in government basically for 50 years. Not a lot of that is sexy, fast, or attractive. But all the other things that have happened to him are — from his stuttering through the death of his wife, to his life in the Senate, the plagiarism issues, Anita Hill and Clarence Thomas — is very intense, very interesting material."

Biography aside, Kirk always knew deep down that, this time, there was every chance The Choice would spring a pre-October surprise.

"For me, it's like trying to write a book that the story keeps changing all the time, and you keep wanting to write 700 pages, but you have to throw the last 200 away constantly because something has changed," he said. "It's a high-wire act…and it has a permanent quality, so you have to get it right.

"If you are doing this long enough, you're always concerned about any of these things that could happen."

One thing about The Choice — and it's especially true this time — is that, unlike breaking news, it doesn't die, or even fade away. It's there for posterity. Kirk knows he has done the job properly if, years or even decades from now, a viewer can watch a Choice from the distant past and relive the moment as if it's happening in the moment.

That's one reason why an objective, bipartisan tone is key to its success. No matter what happens on Nov. 3, the idea is that future generations will be able to view Tuesday's edition of The Choice and understand a little more about what defined Donald Trump and Joe Biden as people, as individuals, and as leaders.

He would not be drawn on whether he admires Trump or Biden.

"I don't do that equation. It is none of my business, actually. I am trying to tell the story as straight as I can.

"If you ask me, what do I think of Trump, I think he's an escape artist. I think he's a fascinating, complicated (person), and that's unbelievably rich territory for a biographer. Is he a political genius? I don't know about that, but he's certainly an escape artist. I don't admire or not admire it. I tell it. I tell the story. That's what I do.

"The difference between the two men is just unbelievable, in all those dimensions. My job is not to say one is good or not, or one is better than the other, or evaluate them in any way. It makes no difference to me, I hope — and to people watching — what I think. It's (about) what tells the story in the clearest, most useful way for people who are trying to decide who to vote for.

"Everybody says elections, campaigns today don't really change the perceptions of 80-, 90 percent of the people, but is that 10 percent for who it really matters."

Kirk has his own ideas about who watches The Choice and what they're likely to take from it.

"(Most) people get confirmation, in terms of what they believe, or they get justification in terms of the negative feeling they might have about somebody.

"But it is that 10 percent — maybe it's nine, maybe it's 13 percent — who really need the kind of information good journalism tries to bring them.

"There is good journalism being done and made in America. Our task here is to deliver information that those people can use."