There’s a bit of a shock when the late John McCain shows up as an interview subject toward the end of Charles Ferguson’s “Watergate,” a comprehensive but frustratingly inessential retelling of how a crime blossomed into a constitutional crisis (you might have read about it). The senator isn’t on screen for long, but it only takes him a few seconds to summarize a profound truth at the heart of this epic documentary: “One thing we politicians are very good at,” he says with a smile, “is kidding ourselves about how well-liked we are.” McCain is talking about Richard Nixon, but — after four hours of watching this film painstakingly connect the dots between then and now — it’s obvious that he’s not only talking about Richard Nixon. Nobody is.

No matter how deep into the weeds Ferguson gets, there isn’t a minute of this movie that...