IT was a slow-burn scandal that began with a very short fuse: Eric Joyce's.

On February 22, 2012, the Falkirk MP threw a drunken punch in a Commons bar that would not only end his own career, but ultimately plunge his party into its biggest crisis in years.

A month after the brawl, Joyce resigned from Labour, but said he would carry on as an independent MP to the 2015 election.

For some it was an insult to his constituents, for others a personal tragedy.

But for Unite, Labour's largest affiliated trades union, it was a golden opportunity.

Just weeks earlier, Labour's biggest donor had adopted an aggressive new political strategy to "reclaim" the party for the working class.

The plan was simple: get Unite members to join Labour in safe seats, then select Unite or Unite-friendly candidates for 2015, thereby changing Labour's make-up at Westminster.

Falkirk, where Joyce had an enviable 7843 majority, and where Labour now needed a new candidate, was irresistible.

For good measure, the chair of Falkirk Constituency Labour Party, Stephen Deans, also happened to be chairman of Unite in Scotland.

What could possibly go wrong?

By last December, the union was patting itself on the back for recruiting 100 new Labour members in Falkirk, a seat with fewer than 200 members.

Unfortunately for Unite, it had also settled on the candidate it wanted to install. Karie Murphy was a disaster waiting to happen. The former nurse from Cambuslang had been a Labour activist for decades, rising to chair of Scottish Labour seven years ago.

However, many of her old colleagues had vivid memories, and when she showed interest in Falkirk, they started talking.

Private warnings were sent to the Scottish Labour leadership.

In 2006, she had faced disciplinary charges at her old union, Unison, after an audit into the NHS branch where she was the secretary.

But Murphy had powerful friends: she was working for MP Tom Watson, Labour's deputy chair, and was close to Watson's old flatmate, Len McCluskey, Unite's general secretary.

She saw Falkirk as her chance of becoming an MP.

In February, the Sunday Herald ran the first of a series of stories about Murphy, Falkirk and the influx of new members signed up by Unite. In a sign of her new clout, our enquiries prompted multiple threats to sue from Murphy's blue-chip lawyers, Mischon de Reya – the firm which represented Princess Diana during her divorce. Unite refuses to say if it paid the bills.

In March, the Sunday Herald reported the first signs of an alleged "stitch-up" in Falkirk.

By then, Murphy had two rivals for the candidacy: former Falkirk Council leader Linda Gow and Gregor Poynton, UK political director of communications firm, Blue State Digital.

Unite helped pay for a survey asking Labour members "do you agree" that the candidate ought to be chosen from an all-women shortlist (AWS). Poynton and Gow wrote to local party members complaining.

Labour halted the consultation and parachuted in a troubleshooter to run the selection.

It caused a local backlash, with the party accused of a "blatant disregard" for democracy.

It also emerged Murphy had promised on Facebook to throw a party when Margaret Thatcher died.

By the end of March, senior Labour sources acknowledged Falkirk was "a gigantic mess". But things only got worse.

In April, former Falkirk provost Dennis Goldie called the Unite-funded AWS survey an "unmitigated disaster", and vice-chair Richard Bryce quit at the "appalling" rancour in his "divided" CLP.

Labour's troubleshooter then contacted new members amid doubts about their "validity".

More than a little local difficulty, Falkirk was taking on a wider significance as a battle between the left and right of the party.

Lord Mandelson chipped in to complain too many selection contests were "in the hands of one union at worst or a couple at best".

On May 17, Labour suspended the selection in Falkirk, saying "concerns" about membership recruitment needed to be investigated further.

Joyce warned he would quit and force a by-election if his successor was chosen in a "fix".

On May 26, the Sunday Herald revealed Labour was investigating claims people had been signed up as Labour members without their knowledge. Among them was a family of three who said they had been recruited even though one only member had agreed in principle to join Labour.

In June, Labour's National Executive Committee took over the local party in Falkirk after its investigation confirmed doubts on the legitimacy of some of the new recruits, and barred all members who joined after March 12, 2012 from voting in the candidate selection.

It was the cue for full-blown civil war. Complaining bitterly that it had been denied sight of the report, Unite claimed the process had been "driven by Blairite pressure" to deny trades unionists influence in the party.

McCluskey threatened to challenge Labour's decisions "by legal action if necessary".

The feud finally became a crisis this week with David Cameron bludgeoning Miliband in the Commons over McCluskey's influence, accusing him of being too weak to stand up to his paymaster. Miliband had to act tough or be dogged with the "weak" tag all the way to the election.

After asking Tom Watson to reconsider an offer to resign on Tuesday, Miliband accepted it – some say insisted on it – on Thursday.

Labour also suspended Murphy and Deans.

McCluskey accused Labour of a "stitch up"; Miliband sent Labour's report to the police.

With Miliband apparently setting Labour's house in order, attention now turns to McCluskey.

Joyce last night attacked Unite, saying a small number of its top officials had been "amateur, hubristic and irresponsible" over Falkirk.

Its high-minded political strategy had been used as cover "to place middle-class officials and close friends into jobs", he said.

Jerry Hicks, who stood against McCluskey for the top job at Unite in April, said: "Without question there is a culture of favouritism in Unite. Len McCluskey is fighting for his own credibility. I think he should be considering his own position."

McCluskey insisted there was no wrongdoing by Unite, and demanded Miliband "step back from the brink of a ruinous division" and stop "playing into the hands" of the Tories.