This was a story long thought dead and buried, but the allegation that Ukraine actively worked with US Democrats to damage Donald Trump's election chances in 2016 has been revived by the top prosecutor in Kiev.
President Trump's personal lawyer, Rudy Giuliani, has repeatedly called on Ukraine to investigate claims of collusion in favour of Hillary Clinton and he has been in regular contact with the prosecutor general, Yuriy Lutsenko.
Now Mr Lutsenko has breathed new life into the story, announcing a new investigation, and citing a court ruling that Ukrainians unlawfully interfered in the 2016 election.
Mr Giuliani also wants Kiev to look into claims that former Vice-President and 2020 Democrat contender Joe Biden may have got Ukraine's then top prosecutor fired to help his son's business interests.
Was there collusion?
The collusion narrative is based around the summer 2016 publication of a "black ledger" in Ukraine which showed off-the-book payments to Paul Manafort.
Manafort, 69, was jailed by a US court in March for fraud, in part for his work as an adviser for a pro-Russian political party in Ukraine, before he became President Donald Trump's campaigner manager.
It was the black ledger payment revelations that forced Manafort to resign from the Trump campaign.
Those arguing that there was collusion, among them Mr Giuliani, allege that the ledger might be fake and that it was maliciously leaked after contacts between Mr Trump's Democrat opponents and Ukrainian diplomats.
Ukraine's prosecutor general gave Mr Giuliani's claims a boost this week by announcing that he was investigating Ukrainian MP Sergiy Leshchenko, who has admitted to being behind the publication of some of the pages of the black ledger.
The problem is that the parts of the ledger that mentioned Mr Manafort weren't leaked by Mr Leshchenko.
They were published by an official Ukrainian state body. There's been no credible evidence that the ledger is fake, indeed many of the smaller payments on it have been tracked down and verified.
The MP has hit back, accusing the prosecutor of making Ukraine a bargaining chip in US politics.
Who says there was interference in the US?
The surprise ruling that Ukraine interfered in the 2016 US election came from Kiev's District Administrative Court.
It is supposed to rule only on public matters but is infamous for controversial judgements.
Ruling on alleged interference in a foreign election is far beyond the court's jurisdiction.
"It's nonsensical," says Mykhailo Zhernakov, a former lawyer and co-founder of judicial reform group Dejure.
"There is no such crime of foreign election interference in Ukraine and it's not a criminal court," Mr Zhernakov says.
In recent months the court's judges have:
- Reinstated the head of Ukraine's tax service in his job, despite him being on trial for corruption, which he denies
- Suspended a reforming health minister
- Ruled that the nationalisation of Privatbank because of a $5bn (£3.9bn) hole in its books should be returned to its owners
If interference isn't a crime in Ukraine, quite where that leaves Prosecutor Yuriy Lutsenko's subsequent "new investigation" is unclear.
Why is Rudy Giuliani involved?
Mr Giuliani's questions about alleged Ukrainian collusion have become a useful counterpoint as Washington continues to debate and digest the Mueller report into the Trump campaign and Russia.
He recently cancelled a planned trip to Ukraine, saying he had discovered that Ukraine's next president, Volodymyr Zelensky, was surrounded by "enemies" of Mr Trump.
With Ukraine dependent on US support to stand up to Russian aggression, they were words that sent a shiver through many in Kiev.
Mr Giuliani specifically named Mr Leshchenko - now an adviser to Mr Zelensky, who will be inaugurated as Ukraine's head of state on Monday.
Prosecutor General Lutsenko, meanwhile, is an appointee of the outgoing president and few believe he will survive the change of administration.
Is this the Democrats' version of Trump/Russia?
Not on the evidence so far.
The Mueller report established that Russian state actors attempted to interfere and influence the US 2016 presidential election. It stated that individuals had violated US criminal law through its social media campaigns and the hacking of emails.
The question then became whether any of the Trump team had coordinated with the Russians in these illegal activities. Mr Mueller's final verdict was that he could not prove they had.
Nothing has been produced or even suggested of anything comparable involving Ukraine.
The release of documents by a Ukrainian anti-corruption agency cannot reasonably be equated with a massive illegal email hack.
This story has filled air-time and column inches on US conservative websites and TV channels - but has not yielded anything worthy of serious legal investigation.
What are the allegations against Joe Biden?
Joe Biden is currently the Democrat front-runner to take on Mr Trump in next year's presidential election.
But in the final years of Barack Obama's presidency Ukraine was a key country for the Biden family.
Mr Biden was the administration's point man as the country faced a Russian invasion and annexation - and the vice-president made numerous trips to Kiev.
At the same time, Joe Biden's son Hunter was a well-paid director at one Ukraine's largest energy companies, Burisma.
Burisma had been subject to several investigations and was founded and run by Mykola Zlochevsky, a minister in the deposed pro-Russian government of Viktor Yanukovych.
Clearly the father-son-Ukraine links don't look great, but questions have been raised as to whether Hunter Biden's lucrative job in any way shaped the US's Ukraine policy.
And, specifically, whether it played any role in the sacking of Ukraine's then prosecutor general Viktor Shokin in early 2016.
It's the sort of foreign interference that Joe Biden has proudly claimed credit for.
Last year he told the story of how he used a billion-dollar loan guarantee to force Mr Shokin out.
"I looked at them and said 'I'm leaving in six hours: if the prosecutor is not fired, you're not getting the money'. Well, son of a bitch. He got fired."
At the time it was almost universally reported that Mr Shokin was removed because he was an obstacle to Ukraine's fight against corruption.
The question now being asked by some is whether it was rather because he was investigating Hunter Biden's company Burisma.
Did Ukraine's prosecutor get fired to help Biden's son?
Based on the evidence currently in the public domain the answer is a pretty clear no.
Mr Shokin himself has suggested that his investigations of Burisma were behind his sacking but the available facts do not back him up.
The timeline of cases shows that most of the Burisma investigations were stalled before Mr Shokin became prosecutor general and that when in office he showed no appetite for pursuing them.
"There was no pressure from anyone from the US to close cases against [Burisma founder] Zlochevsky," Vitaly Kasko, Mr Shokin's deputy in the prosecutor's office at the time, told Bloomberg News.
Comments from both the US and the EU before and at the time of Mr Shokin's sacking reflect that they saw him as an obstacle to change.
With no credible evidence that Mr Shokin was really interested in investigating Burisma or that Joe Biden got him sacked for ulterior reasons, this story, like the collusion allegation, remains very much unproven.
Indeed, amid widespread criticism of his conduct in Kiev, Mr Lutsenko this week clarified that he had no evidence of any wrongdoing by the Bidens.
That's not saying we won't be hearing a lot more about them in the coming months.