On July 18, 2016, Michael Flynn, who pleaded guilty, on Friday morning, to one count of making multiple false statements to the F.B.I. while serving as President Donald Trump’s national-security adviser, took to the stage at the Republican National Convention, in Cleveland. Other speakers that day had included Willie Robertson, of “Duck Dynasty,” and the actor Scott Baio, but Flynn was in what passed as the marquee part of the lineup, following Senator Jeff Sessions, of Alabama (now the Attorney General); Senator Tom Cotton, of Arkansas (now in line to be Trump’s C.I.A. director, if Trump shuffles his Cabinet); and Melania Trump (who, later that night, recited lines from an old Michelle Obama speech). It was, in a word, where you wanted to be if you had enough of a résumé—Flynn was a retired lieutenant general, and a former head of the Defense Intelligence Agency—and wanted to trade your credibility in for a little bit of power in a putative Trump Administration, or for an insider aura to market. (It later emerged that, at the time, Flynn, through his lobbying firm, was collecting money as an unregistered foreign agent for Turkey.) And yet Flynn, in his speech, came across as a true believer and an angry ideologue—to an extent that was electrifying for the crowd in the arena, but also unnerving.

In the middle of a riff about how Obama had supposedly “concealed” the crimes of Osama bin Laden, Flynn said that we did not now need “a weak, spineless President . . . a reckless President who believes she is above the law.” Then, responding to some shouts, he began leading the crowd in the chant of “Lock her up!”—a moment that greatly accelerated the use of that slogan by the Trump campaign. Flynn continued, shaking his fist, “You know why we’re saying that? We’re saying that because if I, a guy who knows this business, if I did a tenth, a tenth of what she did, I would be in jail today.”

The charge to which Flynn pleaded guilty on Friday morning carries a possible sentence of five years in prison. It won’t necessarily come to that, though, because the plea is part of a deal with the special counsel, Robert Mueller, who is investigating possible Russian interference in the 2016 Presidential election. In a statement that Flynn put out at around the time he appeared in a federal court in Washington, D.C., he said that he had made a “decision to cooperate,” having determined that that would be “in the best interests of my family and my country.” He said that he had been wrong to lie to the F.B.I., and that “through my faith in God, I am working to set things right.” But he also added a note of self-justification, and indeed bitterness, prefacing his admission of responsibility with a note that “after 33 years of military service to our country,” he found it “extraordinarily painful to endure these many months of false accusations of ‘treason’ and other outrageous acts.” The F.B.I. agents he lied to were serving their country, too.

The national-security adviser, in any Administration, is a senior figure, clearly more so even than Paul Manafort, who had been Trump’s campaign chairman but was fired before the election, and more than Manafort’s deputy, Rick Gates, whom Mueller has indicted for various financial crimes (both men have pleaded not guilty), and certainly more so than George Papadopoulos, a campaign adviser, who took a plea deal. That Mueller would treat Flynn as someone worth flipping, presumably in pursuit of a bigger case, is, to say the least, suggestive. This is the beginning of a new, significant chapter in the story of the magnetically charged interaction of Michael Flynn and Donald Trump.

The charging documents outline false statements that Flynn now admits having made during an F.B.I. interview on January 24, 2017, four days after Trump’s Inauguration, and after Flynn had accepted a position of national trust. There were at least four lies about two late-December meetings with Sergey Kislyak, the (now former) Russian Ambassador. The substance of those lies is striking: Flynn had denied to the F.B.I. trying to get Kislyak to take positions, on Russia’s response to sanctions and on a Security Council vote regarding Israeli settlements, that ran counter to the foreign-policy efforts of Barack Obama, who was still President.

White House officials, in comments to reporters, presented Flynn’s transgressions as “isolated.” However, Flynn also signed a “statement of offense,” as part of the deal, that described the Kislyak meetings not as freelancing but as something that he was “directed” to do by the members of the Trump transition team. One is identified as a “very senior member” of that group. That sounds like a rich area for investigation.

There have been further indications that Mueller could have thrown more at Flynn if he wasn’t getting something valuable from him. In addition to failing to disclose all sorts of contacts with foreign interests, mostly through his consulting firm, there have been reports, notably in the Wall Street Journal, that Flynn, as part of his paid work for Turkish interests, met with Turkish government representatives to explore how to get a Muslim cleric, who is at odds with the Erdoğan government and lives in Pennsylvania, out of the United States and into Turkey’s custody—one way or the other. (Flynn, through a lawyer, and Turkish officials have denied the Journal’s suggestion that they considered extrajudicial means.) Lying to the F.B.I., as serious as that charge is, rather pales next to a possible kidnapping conspiracy. Indeed, Trump’s choice of a national-security adviser who was reckless or venal enough to even sit down for such meetings is both shocking and, for this White House, typical.

And for Flynn, personally, his own jeopardy may have paled next to the knowledge that his son, Michael Flynn, Jr., who worked with him in his consulting business, is also in legal trouble. It is not yet clear whether his deal benefits the younger Flynn. But the elder Flynn, in the end, may simply have decided that he was more loyal to Michael, Jr., than to Donald, Sr.

One question Mueller is no doubt exploring is whether Trump’s own efforts to help Flynn—including asking James Comey, then the F.B.I. director, to give Flynn a pass, and then firing Comey after, among other things, it became clear that that wasn’t happening—were motivated by loyalty or by apprehension about what that investigation might reveal. This is the obstruction-of-justice question. It is the one that at least sets the framework for impeachment, even if the political realities make that scenario improbable for as long as the House and the Senate remain in Republican control.

Flynn, in his Convention speech, moved on from the shouts about locking up Clinton to urge the crowd to take up another chant: “Trump! Trump! Trump! Let’s go. Come on.” He also said, “Let us not fear what we know to be true . . . wake up, America!” Flynn had lost his job as head of the D.I.A. out of a sense, among other things, on the part of Obama officials who dealt with him, that he had succumbed to dark views of hidden forces, particularly about Muslims. (This is a tendency he also revealed in his retweeting habits—a tendency Trump shares, as this week made clear.) Flynn added, “You cannot sit this one out. You cannot sit this election out. Get out of your houses and get out there and vote.” The truth there is that, whatever Flynn’s crimes, extremism is not something, ultimately, that can be countered with a special counsel—people do need to get out and vote. Even before his legal problems emerged, Flynn embodied the way that, in Trump’s orbit, conspiratorialism was intertwined with real influence. That story, too, isn’t over.