The stakes are high with the next presidential election 16 months away.
With a huge national television audience expected, Democrats want Mueller to help swing public opinion against the president, with some lawmakers hoping he will provide more firepower for an impeachment case against Trump.
But the former FBI director's deep reticence to become embroiled in politics, and Republican plans to turn the hearings into a reality TV-like partisan brawl, threaten to leave Americans no less confused than when Mueller's dense, 448-page report was released in April.
"The public has a right to hear the truth, from Mueller himself, about Trump's misconduct and ongoing national security risks," said Adam Schiff, chair of one of the two House committees hosting Wednesday's marathon testimony.
Was Trump exonerated or not?
The final report of Mueller's investigation detailed extensive Russian interference in the 2016 election to boost Trump, and multiple instances of attempted collusion between the campaign and Moscow's agents.
But in the end Mueller, a veteran federal prosecutor, found no crime in that -- simple collusion is not an criminal offense, and his team concluded there was not enough evidence to support charges of criminal conspiracy with the Russians.
They also enumerated at least 10 instances where Trump sought to impede the investigation, with strong evidence in support of obstruction of justice allegations.
But Mueller declined to recommend obstruction charges, saying he was blocked from doing so by Department of Justice policy that says a sitting president cannot be charged.
That allowed Trump to declare that he was "exonerated" by Mueller and say the investigation was always a political "witch hunt," claims that have gone far in convincing much of the public that it was much ado about nothing.
"This Witch Hunt must now end. No more Do Overs. No Collusion, No Obstruction. The Great Hoax is dead!" Trump tweeted in early July.
Margaret Taylor, a governance expert at the Brookings Institution think tank in Washington, said the hearings are an opportunity to clarify Mueller's findings.
"I don't think that Americans fully understand the systemic attempts by the Kremlin to interfere, and how the Trump campaign reacted to that," she said in an interview.
"The vast majority of Americans did not read the report," she said.
"I think there is a large portion of the country who hears the president say 'no collusion, no obstruction' and that's the end of the story for them."
Challenge getting Mueller to talk
The much-anticipated hearings will unroll on Wednesday, starting at 8:30 am (1330 GMT) before the House Judiciary committee and then moving to Schiff's House Intelligence Committee at 12 noon.
Both parties were reportedly rehearsing their tactics in the week before to blunt each other's efforts.
Democrat and Republican committee members will likely alternate for five minutes of questions each, a format which will challenge Democrats to deliver to the television audience a coherent narrative of collusion and obstruction, while Republicans try to disrupt it and change the subject.
Democrats need to figure out how to get the cagey and disciplined Mueller to say in clear terms that Trump likely broke obstruction laws, while Republicans are expected to try and undermine Mueller's credibility and the original basis of the investigation.
Mueller, much-trusted in Washington circles but with a sphinx-like image, has already signalled he will be a tough nut to crack.
"Any testimony from this office would not go beyond our report," he said on May 29, as he announced the closure of the special counsel's investigation.
"The work speaks for itself. And the report is my testimony."
"What he won't do is go beyond the four corners of the report, as he's already said," former FBI deputy director Andrew McCabe, who has worked closely with Mueller, told CNN.
But McCabe said that cannily phrased questions could draw Mueller out -- especially on the issue over whether he personally believes Trump broke the law.
"So if that's the place that Congress wants to get to, there are many, many ways they can get very close to that," he said.
Some speculate that Mueller, 74, might be ready to purge some of his feelings after repeated personal attacks from Trump and Trump-allied Attorney General Bill Barr's partisan dismissal of his findings.
"If he decides that he has got to the end of his career and he is irritated with how this report has been handled, then maybe he'll be more loquacious," said Taylor.