It’s been a long year since Trump’s former national security adviser Michael Flynn agreed to cooperate with special counsel Robert Mueller’s investigation — and in that time, we’ve gotten no confirmation of what that cooperation entailed.

But on Tuesday, Mueller’s office is due to file a sentencing memo in Flynn’s case, which could give some sense of just how valuable they think his information has been.

Last December, Flynn admitted lying to the FBI about his contacts with Russian Ambassador Sergey Kislyak. Back then, he was the second former Trump adviser, after George Papadopoulos, to cut a deal with Mueller’s team.

Much has happened in the investigation since then — though very little, that we know of, that has anything to do with Flynn. Another Trump campaign aide, Rick Gates, flipped in February. Mueller indicted more than two dozen Russians for social media propaganda, hacking, and attempted witness tampering. Trump campaign chair Paul Manafort was convicted of financial crimes, flipped, and then un-flipped. And former Trump attorney Michael Cohen also struck a plea deal just last week.

So far, though, Flynn’s cooperation remains mysterious. It does not appear to have any obvious connection the other developments in the investigation and has not led to any known charges. It’s unclear what Mueller’s team has been doing with him these past 12 months — and whether it’s led anywhere.

We do know that, in several filings throughout this year, Mueller said Flynn was not yet ready for sentencing “due to the status of” the special counsel investigation. It took until September 17 for Mueller to say Flynn was ready for sentencing. Something was clearly going on in the interim.

But what? Today’s filing could give us some more information.

What did Michael Flynn plead guilty to again?

Back in December 2016, Donald Trump had just won the presidential election and named Flynn as his national security adviser-designate. The retired lieutenant general had advised Trump during the campaign (famously saying “Lock her up!” about Hillary Clinton during a speech at the GOP convention). So he was rewarded with one of the most important foreign policy jobs in the administration.

During the transition period, however, Flynn had some curious contacts with Sergey Kislyak — Russia’s ambassador to the US.

First, on December 22, 2016, Flynn reached out to Kislyak because the United Nations Security Council was planning to vote to condemn Israel’s settlement policy. Flynn urged Kislyak to get Russia to block the resolution, but he refused.

Second, and more significantly, after President Barack Obama announced new sanctions on Russia in response to their interference with the 2016 election, Flynn reached out to Kislyak at the behest of Trump’s transition team. He urged Kislyak that Russia should refrain from retaliating against the US.

Putin took Flynn’s advice — the next day, he released a statement saying he wouldn’t retaliate. Then, US president-elect Donald Trump tweeted this out:

Great move on delay (by V. Putin) - I always knew he was very smart! — Donald J. Trump (@realDonaldTrump) December 30, 2016

Questions immediately arose over whether the incoming Trump administration was trying to undercut Obama’s new sanctions, perhaps in gratitude for Russia’s election meddling.

At the time, Flynn’s contacts with Kislyak were secret — but they didn’t remain secret for long. In mid-January 2017, news leaked that Flynn and Kislyak had been in contact on the day of the sanctions announcement. It later emerged that Kislyak was under surveillance, and Flynn’s contacts with him were picked up.

Key Trump advisers like incoming press secretary Sean Spicer and vice president-elect Mike Pence both insisted publicly that, according to Flynn, the topic of sanctions didn’t even come up between him and Kislyak. That’s where things stood when Trump was sworn in as president, and Flynn became national security adviser.

Just a few days later, though, two FBI agents (including Peter Strzok) paid Flynn a visit. They asked him about his talks with Kislyak, and he again denied talking about sanctions with the ambassador. He also denied lobbying Kislyak on the UN Security Council vote. Intelligence intercepts made clear that both those claims were false, and acting attorney general Sally Yates warned the White House that Flynn had lied.

Trump took no action against Flynn for 18 days after Yates’s warning. Only after a pair of Washington Post reports publicly revealed, first, that Flynn and Kislyak discuss sanctions, and second, the warning from Yates, did Trump finally fire Flynn (on the night of February 13). The White House claim was that Trump ousted Flynn because he had misled Pence.

It also later emerged that, the day after Flynn’s firing, Trump asked all other officials to leave a meeting so he could speak with then-FBI Director James Comey alone. Per Comey, Trump told him, “I hope you can see your way clear to letting this go, to letting Flynn go.”

The whole thing seemed deeply strange. Why had Flynn lied? Why did Trump take so long to fire him? Why did the president want to the FBI director about his case in private? Was all this suggestive of a larger Trump-Russia scandal?

Once special counsel Robert Mueller was appointed in May 2017, his team further investigated Flynn — not just for his false statements to the FBI, but also for other shady-looking business and foreign dealings.

In the end, though, Flynn cut a deal. He admitted lying to the FBI about his conversations with Kislyak — and he agreed to cooperate with Mueller’s probe. Toward what end, we don’t yet know.