All eyes will be on the committee rooms of Palace of Westminster, as Cummings takes center stage, facing questions from MPs at a parliamentary inquiry into how Johnson and his team dealt with the coronavirus pandemic.
These hearings are usually bland, procedural affairs, but this one may become a key moment in Johnson's premiership.
Not long ago, Johnson and Cummings were thick as thieves. Johnson had Cummings to thank -- at least in part -- for his two greatest political triumphs: Brexit and his landslide election victory in 2019.
Cummings is one of the most controversial figures ever to have entered British politics, and his combative style became a defining feature of Johnson's government.
He famously hired a subordinate -- through his personal blog -- who had previously commented publicly in support of eugenics.
He demanded total control of the government's network of advisers, resulting in someone he held responsible for a leak to the press being escorted out of Downing Street by armed police.
His loathing of the British political establishment led numerous high-ranking Conservatives to warn Johnson against taking him into government -- not least because Cummings himself was not a Conservative and has openly expressed contempt for members of the party.
Johnson -- convinced as he was of Cummings' genius -- ignored that advice. Unfortunately for the Prime Minister, it didn't end well.
Following a bitter argument at the heart of Johnson's inner circle, Cummings resigned last November. Photographs of him departing Number 10 for the last time, carrying a cardboard box, led to wild speculation as to what might be inside.
On Tuesday, Cummings offered us a clue, posting in a since-deleted tweet that he was in possession of the only copy of a "crucial" document on the government's handling of Covid.
In other tweets posted at around the same time, Cummings extolled the value of hard lockdowns to help prevent deaths from the coronavirus.
The context here is important. Last month, numerous UK media outlets ran stories claiming Johnson had said he would rather "let the bodies pile high in their thousands" than impose another national lockdown in October 2020. The comments, which Johnson denies making, allegedly came after he had reluctantly agreed to a second lockdown -- the implication being that this would be the final one. Another lockdown was introduced in January 2021.
The opposition Labour party claims Johnson's delays caused more sickness, death and misery. "What their approach led to was more people getting ill, followed by deeper and longer lockdowns, that then did far more damage to the economy," a spokesperson for the party told CNN. Numerous epidemiologists have agreed with this and it's an undeniable fact that the UK had a very bad economic downturn, even compared to other major economies.
Many believe Cummings was the source behind that damaging "bodies" allegation -- a claim he denies.
But Cummings has publicly attacked his former boss, along with members of his inner circle, including Johnson's fiancé, Carrie Symonds, who is alleged to have been a key figure in the factional fighting within Downing Street that led to Cummings' departure from government after she apparently blocked a promotion of a key Cummings ally.
Downing Street declined a request to comment on this matter.
Which is why many in government fear Cummings will use his moment in the spotlight next week to inflict maximum damage.
In order to get the broadest picture of what allies of both Johnson and Cummings believe could happen on Wednesday -- and how significant it will be -- CNN spoke to more than a dozen current and former officials who served Johnson, Cummings or both. They all agreed to speak candidly, on the understanding that they would not be named in this article.
"The danger zone is what happened in the run up to that Autumn lockdown," says one former Downing Street official. "The data clearly showed in September a spike was coming for weeks, but we didn't go into lockdown until November. If Dominic has evidence the PM dragged his feet then he could be in trouble."
"Having Dom go completely off the rails is a nightmare, as the government cannot really fight back," says another former adviser, adding that "it would look too undignified" coming from the people who are supposed to be leading the nation through a crisis.
Some inside Number 10 are worried that Cummings has audio recordings of the PM saying "inappropriate stuff" that could be taken out of context.
A former colleague of both Johnson and Cummings explains that the PM "does have a habit of saying things that could look bad out of context, like comments about individuals or policies."
And there are fears, too, that Cummings will go after Johnson and the people around him on issues beyond the government's handling of the pandemic.
CNN approached Cummings for comment but he declined.
In a blog post published last month, he claimed Johnson had tried to use Conservative party donors to pay for the refurbishment of his flat in Downing Street. Cummings said he'd told Johnson his actions were "unethical, foolish, possibly illegal and almost certainly broke the rules on proper disclosure of political donations."
The former adviser also alleged that Johnson had tried to stop an inquiry into government leaks, for fear it might implicate a confidante of Symonds. "I told him that he could not possibly cancel an inquiry about a leak that affected millions of people, just because it might implicate his girlfriend's friends," Cummings wrote.
Johnson indirectly denied any wrongdoing on both of these accusations
On paper this might look like little more than palace intrigue, but such allegations could seriously hamper the ability of senior officials to do their job. And it all adds to the whiff of sleaze that has come to surround Johnson in recent weeks.
Some advisers admit they are "freaked out" at the thought that Cummings is going to go after their credibility.
The situation is probably not helped by the fact that Cummings' allies are rubbing their hands with glee at the prospect of him getting his own back on Johnson.
The rift that led Cummings and many of his staff to quit was seriously unpleasant, and many of those loyal to him still feel aggrieved that Symonds' friends won the battle.
But others in government are calmer and keen to paint Cummings as a somewhat tragic figure.
'Unpopular and discredited man'
The reason most people will have heard of him is not because of his work on Brexit -- or any other policy, for that matter -- but because of a scandal at the peak of the first wave of the pandemic, when Cummings drove his wife and child hundreds of miles across the country during a national lockdown after coming down with Covid-like symptoms.
After the news broke, Cummings gave a press conference in the Downing Street garden in which he came across as arrogant and unapologetic. Many thought he should be sacked for breaking the very rules he had helped to write.
According to Ben Page of public research firm IPSOS Mori, "Cummings was the number one reason people gave when explaining why they broke lockdown restrictions. He is a very unpopular and discredited man."
"I think he might be one of the least popular people in the country," a government minister told CNN. "The main thing I will be watching for is if he has any new information which points to his remaining allies in government still leaking to him."
Meanwhile, Johnson is currently riding high after the monumental success of the UK's vaccine rollout. His Conservative party won seats in a number of elections across the UK earlier this month, including extending their Westminster majority by one.
Other senior government sources say the ongoing unpleasantness between Johnson and Cummings is a sad end to a partnership that was so successful.
"He is clearly a ridiculously smart person, but he's not using it for good anymore," said one. "Unfortunately for Cummings, unless he's got very hard evidence of wrongdoing, people will probably see him as a bitter former employee. Our best response is to get on with delivering the vaccine -- tangible things the public can see as promises kept."
While Johnson's popularity, relative to Cummings, does place the latter at a disadvantage, there is a risk in the government appearing too complacent.
"This is a man who sat at the very heart of the decision-making processes around one of the biggest crises this country has faced," says Lauren McEvatt, a former Conservative adviser. "The idea that he has nothing to say that is of value, or indeed nothing that could potentially damage the government, is naïve."
She concedes, though, that Cummings' reputation "may work to undermine him."
The other concern for Johnson is that Cummings doesn't really need to come out of Wednesday well. "If all he wants to do is make Boris look bad, he doesn't stand to lose very much," says one recently departed official.
McEvatt points out that "it would be a disservice to this country if the committee, or Dom, chose this as an opportunity for cheap point-scoring."
Yet anyone who has kept an eye on British politics over the past five years will be only too aware that cheap point-scoring isn't beneath many in Westminster.
It's most unlikely that Cummings' evidence will have much material impact on the day, but there is a real chance it could play into a bigger picture at some point in the future.
A public inquiry, which Johnson has said will begin in spring 2022, may find that the Prime Minister failed at key moments. Perhaps his personal conduct in office will look much worse in two or three years.
Cummings may not have a smoking gun right now, but he appears angry enough to want to cause Johnson as much damage as possible in a very public setting.
All of which means that -- whether he brings solid evidence or not -- Wednesday is unlikely to be an edifying spectacle; keen Westminster watchers will be stocking up on popcorn in readiness.
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