The one familiar aspect of the David Petraeus scandal is that he had an affair. Everything else about this story is weird.

Petraeus, who resigned Friday as director of the CIA, is brilliant, brave, dedicated and accomplished. But he is also vain. Even his most loyal and ardent supporters have to acknowledge the care with which he has always burnished his own image. He is used to being surrounded by acolytes — staff officers, journalists, hangers-on — whose fawning attentions can only foster a sense of superiority and entitlement.

Not every man in that situation betrays his marriage vows. Some do, as evidenced by the whole of human history.

So the sex part is deplorable but comprehensible. The rest of this saga is bizarrely opaque, starting with the timing.

According to reports by The Washington Post and other news outlets, the FBI investigation that uncovered the relationship between the retired four-star general and Paula Broadwell, his two-decades-younger biographer, was launched early in the summer. Yet Petraeus’ boss, Director of National Intelligence James Clapper, wasn’t told of the inquiry until Nov. 6. Which just happened to be Election Day.

Sorry, but there are no coincidences in spy novels.

It is inconceivable that FBI agents would skulk around investigating the private life of the CIA director without informing top officials of the bureau and the Justice Department. It was obvious that as soon as Clapper knew, he would have to inform President Obama — and that Clapper would have to make some recommendation about Petraeus’ future. The whole mess surely would come to light.

Tell you what: If the right-wing conspiracy theorists will acknowledge that the scandal wouldn’t have materially affected the outcome of Tuesday’s vote, I’ll admit that it sure looks as if someone decided to keep the White House in the dark until the political season was over.

Conspiracy buffs should also acknowledge that the scandal’s timing could not possibly have been a way to keep Petraeus from testifying on Capitol Hill about Benghazi. Congress can still call him to the witness table anytime it chooses.

It’s possible that the FBI kept the investigation such a closely held secret because it wanted to avoid the perception that the bureau was somehow going after the CIA. Then again, I guess it’s possible that the bureau was going after the CIA.

Another mystery is why the nation’s chief spy didn’t practice better tradecraft in seeking to protect his little secret.

According to widely published reports, Petraeus carried on a steamy email correspondence with Broadwell through a private Gmail account that he opened using a pseudonym.

It’s as if he didn’t know anything about IP addresses or location data. Presumably, as CIA chief, Petraeus must have read intercepted emails sent by terrorists who tried to disguise themselves by using false names. Why did no light bulb appear above his head, no thought bubble saying, “Gee, even if I don’t use my real name, somebody might figure out it’s me”?

We know a bit about Broadwell, the West Point graduate whose book about Petraeus was, to say the least, quite positive. When she appeared on “The Daily Show” in January, Jon Stewart observed, “The real controversy here is, ‘Is he awesome or incredibly awesome?'”

But we still know very little about Jill Kelley, who has been called “the other other woman” but who might also be just an innocent bystander. It was Broadwell’s string of emails to Kelley, apparently accusing her of having or seeking an affair with Petraeus, that launched the investigation.

We know that Kelley lives in Tampa, is married, and that she and her husband count David and Holly Petraeus among their friends. It is unclear whether she knows Broadwell, and it appears that the menacing emails she received had been sent anonymously.

In the end, this may be a simple story: A woman gets a series of disturbing messages and asks an FBI agent she knows for help. A few months later, the nation’s chief spy — and perhaps its greatest living military hero — comes crashing down.

If you believe in coincidences.

If you don’t, there has to be a foreign spymaster involved, an updated version of John le Carre’s diabolical Karla, an unseen figure manipulating these characters like puppets toward subtle and devious ends. Maybe I see the game that’s being played. Maybe I know what this is all about.

But if I told you, I’d have to kill you.

Eugene Robinson’s e-mail address is eugenerobinson(at)washpost.com.