Here's a look at the key stories from the investigation, which are featured in the documentary.

Trump Tower meeting revealed

In the months after the election, the Trump team denied that there were any contacts with Russians during the campaign. Trump said so himself at a news conference in February 2017. "I have nothing to do with Russia," he declared. "To the best of my knowledge, no person that I deal with does."

That's why it was so shocking when The New York Times dropped this bombshell over the summer: The President's son, Donald Trump Jr., met with a Kremlin-connected lawyer during the campaign after being promised dirt on Democratic nominee Hillary Clinton. Not only that, but Trump Jr. was joined by Trump's son-in-law Jared Kushner and Paul Manafort, who was the campaign chairman at the time.

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"When Donald Trump Jr. is told the Russian government is trying to elect your father president, he doesn't say, 'what do you mean? How can that be?' He says, 'I wanna hear this'," said CNN's chief legal analyst Jeffrey Toobin, referring to the emails sent to Trump Jr. before the meeting

Special counsel Robert Mueller's team started investigating the meeting. It also posed a political problem for the White House.

"You're saying constantly there was no effort to collude with the Russians in any way, shape or form, and suddenly, you have your son, your campaign manager, and your senior adviser all in a meeting with Russians who have promised to bring you dirt," said CNN's chief political analyst Gloria Borger.

Trump Jr., Kushner and Manafort have all denied participating in any collusion. Russian lawyer Natalia Veselnitskaya has told conflicting stories about the purpose of the meeting and her Russian government connections.

James Comey fired

"This is not normal, this is not how presidents behave. It's a dark moment in American history today."

That's how Toobin reacted on May 9, 2017, the day Trump unexpectedly fired FBI Director James Comey , the man who was overseeing a federal investigation into Trump's presidential campaign.

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Everyone was caught off guard, including senior White House aides. The FBI director himself learned about his fate while in California on official business by watching it on television.

Trump later admitted he was thinking about Russia when he decided to fire Comey, seeming to contradict the official White House line that Comey was fired for mishandling the Clinton email investigation.

An action perhaps designed to neutralize the Russia investigation instead triggered a series of events that led to Mueller's appointment as special counsel. "That just gives you a sense of how impulsive this firing was and it really did backfire," said CNN's chief political correspondent Dana Bash.

After his firing, Comey stayed in the limelight. He starred in a highly anticipated hearing before the Senate Intelligence Committee, which was aired live on the networks and shown at bars throughout Washington. And in the future, he might find himself on the witness stand, if Mueller sees fit.

Scrutiny of top Trump aides

Three men close to Trump -- Kushner, Manafort and former national security adviser Michael Flynn -- all got entangled in the Russia probe last year.

Kushner went before congressional committees investigating potential collusion, and was asked to turn over documents to the special counsel. He denies knowing about any collusion with Russia, though Mueller's investigation continues to scrutinize his activities, with no clear end in sight.

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"Jared Kushner is incredibly important for several reasons," Toobin said. "He is a witness to virtually all the central issues in this investigation. He is also important because he is an independent actor. He is someone who had contacts with Russian representatives."

Then there's Flynn, who pleaded guilty in December to lying to the FBI and is now cooperating with Mueller. He started 2017 as the incoming national security adviser -- a plum job for someone who had been previously fired from a high-ranking intelligence post during the Obama administration.

And finally, Manafort is slated for the fight of his life in 2018. Manafort and co-defendant Rick Gates, his longtime business partner and deputy on the Trump campaign, go on trial this spring. They pleaded not guilty to a dozen federal charges of money laundering, undisclosed foreign lobbying, and conspiracy against the United States.

"Paul Manafort has, for decades, been the Washington lobbyist and influence peddler around the world for a lot of the most vicious dictators we've seen," said investigative reporter and Pulitzer Prize winner David Cay Johnston.

Business deals with Russians

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Trump's ties to Russians go back way before the election. In fact, he's been trying to do real estate deals in Russia since the 1980s. In recent years, he partnered with a wealthy Russian on the 2013 Miss Universe pageant, which was held in Moscow, and sold a Florida mansion to a Russian in 2008.

"We know that there's plenty of Russian money that the President has benefited from," said CNN justice correspondent Evan Perez. The Florida estate alone was sold for about $95 million

Not only that, but the Trump Organization also negotiated with a Russian company during the presidential campaign to build a Trump Tower in Moscow. Trump even signed a letter-of-intent , but the deal fell through in early 2016, a few weeks before the first primary contest in Iowa.

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It is yet to be seen how aggressively Mueller will examine these old business deals that predate the election. Trump and his lawyers have made it known that they don't want Mueller to cross that line.

Russian meddling on social media

While the Russians disrupted the presidential campaign through hacked emails and high-profile leaks, they were also active on a more granular level that affected millions of Americans: social media.

It took months to learn the full scope, which still might be unknown, but leading US technology companies like Google, Facebook and Twitter now acknowledge that Russians were active on their platforms during the election, creating fake accounts and stirring up political tensions on controversial topics.

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"There was an entire English language department specifically assigned to insert messages, social media posts in the United States," Michael Isikoff of Yahoo News said about the Russian misinformation campaign. "They were required to watch 'House of Cards' to better understand American politics."

Some of the Russian trolls even organized in-person protests on US soil.

"You may very well have driven by a protest for any hot-button issue," said CNN's Dylan Byers, senior reporter for media and politics. "These protests in some cases were organized out of Russia."

CNN led the way reporting on the social media element of Russian interference, exposing fake accounts linked to the Kremlin, revealing Russian efforts to target specific US communities, and uncovering that some key swing states also were subjected to Russian-backed Facebook ads.

The dossier

This bombshell, one of the biggest of the year, was broken by CNN weeks before Trump took office: During a briefing by the nation's top intelligence chiefs similar to the one then-President Barack Obama had, Trump was told about a collection of memos, written by a respected former British spy, that alleged widespread collusion between the Trump campaign and the Russians, and that the Russians had compromising personal information on Trump.

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"It was our sense -- our strong sense -- that the nation's senior most intelligence officials would not waste the time of the president and the president elect if it was cockamamie easy to dismiss information," CNN's chief national security correspondent Jim Sciutto said. "It's a remarkable step."

The memos came to be known as "the dossier." Shortly after the initial CNN report, the full dossier was published online by BuzzFeed for everyone to read -- even the unconfirmed, salacious allegations.

In the past year, the dossier has become the boogeyman of Republicans and a sacred text for Democrats. Regardless of the partisan spin, CNN has reported that US investigators have corroborated some aspects of the dossier, especially claims relating to conversations among foreign nationals.

It has yet to be seen how many of the allegations in the dossier are true, and if that will even matter. Mueller's investigation has focused on examining the links between the Trump campaign and the Russian government, and his team has already secured two guilty pleas and another two indictments.