On Friday it was Michael Flynn's turn "in the barrel", to borrow a line from Trump confidant Roger Stone. The former national security adviser pleaded guilty to lying to the FBI about December 2016 conversations he had with Russian ambassador Sergei Kislyak and pledged to "fully cooperate" with Mr Mueller's ongoing investigations.
Mr Flynn has admitted he misled the FBI about his discussions regarding new sanctions the Obama administration imposed on Russia following evidence of alleged meddling in the 2016 election.
There had been hints this was coming, after word last week that Mr Flynn's defence lawyers had stopped co-operating with the Trump legal team. The president's own scattershot behaviour on Twitter this week could also have been a key tell, like a trick knee acting up before a big storm.
So why is this being billed as a major development in the ongoing investigation into possible Trump campaign ties to Russia? Let us count the ways.
1) Trump's inner circle has been breached
It's difficult to overstate the significance of this felony plea deal. Mr Flynn was a close adviser and confidant of Mr Trump throughout the 2016 presidential race. He was a surrogate for the candidate on television and enjoyed a prominent speaking role at the July Republican National Convention. He had a pivotal role in Mr Trump's presidential transition.
The role of national security adviser in the White House, which Mr Flynn assumed upon Mr Trump's inauguration, is one of the senior-most positions in any administration, responsible for being the key conduit between the sprawling US military and intelligence bureaucracies and the president. It is a post that has been held by the likes of Henry Kissinger, Zbigniew Brzezinski, Colin Powell and Condoleezza Rice.
Mr Trump was so partial to Mr Flynn that he was praising him as a "wonderful man" who had been "treated very, very unfairly by the media" just days after firing him.
Now Mr Flynn could be going to jail - and, more importantly, could be sharing damaging information about the Trump inner circle he inhabited for so long.
2) Flynn is talking
According to the Statement of the Offense filed by the special counsel's office, Mr Flynn is testifying that he had contact with Trump transition team officials before and after his fateful December 2016 conversation with Ambassador Kislyak. "Members of the transition team," the document relates, "did not want Russia to escalate the situation after the Obama administration imposed new sanctions on the Russian government".
These conversations came more than a month after Mr Trump had won the presidency. Mr Flynn had already been announced as the national security adviser in the incoming White House - a top post in the president's inner circle.
The next big question is who exactly were the unnamed senior members of the presidential transition team. Some US news outlets are naming Jared Kushner and former Deputy National Security Adviser KT McFarland. Others seem to indicate it was Mr Trump himself. Eventually, Mr Flynn - and Mr Mueller - will have to lay their cards on the table.
3) Flynn is contradicting the White House line
Mr Flynn's assertions about his conversations with the transition team run directly counter to statements made by Mr Trump in a February press conference, where he said Mr Flynn was acting against orders when he reached out to Mr Kislyak.
In fact, the White House said at the time that the president dismissed Mr Flynn as national security adviser because he lied to Vice-President Mike Pence about his Russian contacts. (The true nature of Mr Flynn's conversations with Mr Kislyak first came out thanks to information gleaned from government surveillance of Mr Kislyak that leaked to the press).
If Mr Flynn has evidence corroborating his account of December contacts with the Trump transition team - which was headed by Mr Pence himself - the White House's explanation for its handling of the Flynn situation, denials of knowledge and all, start to crumble.
Anyone in the president's inner circle who told the FBI or Mr Mueller's investigators that they weren't privy to Mr Flynn's activities, but there is evidence that they knew, would be open to another round of charges of lying to the FBI.
The White House response, at least so far, seems to be that Mr Flynn is a lying liar who lies.
"The false statements involved mirror the false statements to White House officials which resulted in his resignation in February of this year," White House lawyer Ty Cobb wrote in a press statement. "Nothing about the guilty plea or the charge implicates anyone other than Mr Flynn."
4) Mr Mueller could be building an obstruction of justice case
Dust off that old political saw that "it's not the crime, it's the cover-up". While Mr Flynn's contact with the Russian ambassador is questionable, given that he was undercutting Obama administration policy efforts, it's probably not illegal.
What is illegal, however, is obstruction of justice. Former FBI Director James Comey has testified that on 14 February - the day after Mr Flynn was sacked - Mr Trump urged the director to back off his investigation into Mr Flynn during a private Oval Office meeting.
If the president knew that the ongoing law-enforcement inquiry would discover Mr Flynn had been acting under orders - either by the president or a member of his transition team - that could be the kind of motive that would help support an obstruction of justice charge.
5) Only the tip of the iceberg?
There were a lot of rumours and allegations floating around about Mr Flynn before Friday's plea deal news. The special counsel's office was reportedly looking into Mr Flynn's Obama-era work as director of the Defense Intelligence Agency. It was scrutinising his 2015 trip to Russia, paid for by the government-backed RT network, and his undisclosed lobbying on behalf of Turkish government interests.
The charge brought against him, however, was solely related to his December 2016 phone conversations with Mr Kislyak. Although it comes with a possible five-year prison sentence, Mr Mueller hardly threw the book at the former national security adviser. Is this all there is?
Mr Mueller is primarily tasked with investigating possible ties between the Trump campaign and the Russian government. Mr Flynn was a senior adviser to and advocate for Mr Trump's presidential bid. Does relative modesty of the charges against Mr Flynn indicate he may be offering information directly relevant to this inquiry?
6) The independent counsel investigation is wide-ranging
Mr Flynn's plea deal is just one piece of a much larger puzzle the special counsel office is trying to solve.
In October Mr Mueller indicted former Trump campaign chair Paul Manafort and Rick Gates, a top aide with White House ties, on money laundering charges predating their involvement with the Trump campaign.
He also struck a plea deal with former foreign policy adviser George Papadopoulos, who told prosecutors he lied about his own contacts with Russians.
Each move is distinct and not directly related - at least not yet. A some point we're going to learn whether Mr Mueller is building a larger case against the Trump campaign out of these legal moves - or that the sum total of his efforts is nibbling around the edges.
As the president likes to say, stay tuned.