Cairns, a former New Zealand cricketer, is on trial on charges of perjury and perverting the course of justice over match-fixing allegations.
In the final flourish of his closing speech to the jury at the trial in Southwark Crown Court, Orlando Pownall, QC, said the Crown case could be summarised with "Why would they all lie?"
But Mr Pownall said that, in reality, the case was nothing but "rumour upon rumour" and a biased investigation that refused to accept any possibility other than Cairns was guilty.
"We invite you to be careful to make no assumptions. Beyond rumour, beyond self-motivated lies, you cannot be sure Mr Cairns is guilty. For that reason, we invite you to acquit him."
McCullum's three statements to investigators from the International Cricket Council and later the Metropolitan Police in London changed significantly, said Mr Pownall.
The barrister said the conversation between Cairns and McCullum in the Kolkata hotel room in 2008 was blown out of proportion over time.
In February 2011, McCullum said Cairns talked about "spread betting" but this escalated to "match fixing" in subsequent statements in 2013 and 2014.
McCullum also did not remember the names of others allegedly involved in match-fixing with Cairns until the 2014 statement.
At that point, he said Cairns "definitely" told him Vincent and Daryl Tuffey were also cheating at the Chandigarh Lions. This was after Lou Vincent had been interviewed by the ICC in September 2013.
"How could he have forgotten?" asked Mr Pownall, pointing out that McCullum would remember the names of fellow New Zealand internationals.
Mr Pownall borrowed a cricketing phrase to say McCullum and Vincent had each scored a century of "I can't recall" when asked difficult questions.
Daniel Vettori also remembered McCullum mentioning in 2008 the names of Vincent and Tuffey in connection to the Cairns allegations.
If that were true, the New Zealand captain knew about a "match-fixer in your midst" - referring to Tuffey, who was still playing for New Zealand - but did nothing, said Mr Pownall.
As captain, Vettori was a selector for the national side and said in evidence that rumours of Tuffey's involvement in match-fixing were discussed.
This was not supported by fellow selectors Mark Greatbatch and Glenn Turner who could not recall the Tuffey rumours being discussed.
At first Vettori said McCullum told him about Cairns in 2010, but later changed this to 2008.
Mr Pownall said the allegation, where the captain of New Zealand team was told by a rising star about a former captain, should be "burnt on the individual's recollection".
"The defence suggests that Mr Vettori has not told the truth, not that he has been an evil purveyor - that would be a mischaracterisation - but for his own benefit, he has seen fit to mislead."
Vettori did not remember the discussion because it was "insignificant", said Mr Pownall, the same reason the ICC did not investigate further.
But McCullum's statement was given greater importance when Vincent "spilled the beans" in 2013.
"If Lou Vincent had not come forward, it would all be buried. Once he came forward, the McCullum account could no longer be ignored," said Mr Pownall.
"Then [McCullum] had choice to make. Leave it in equivocal terms about betting.... Or give a fuller, more detailed account."
McCullum and Vettori essentially tailored their evidence to escape a potential playing ban which could have affected the Cricket World Cup played on home soil this year, said Mr Pownall.
Similarly, Vincent's former wife Ellie Riley gave evidence only after the British police ruled out laying charges against her.
Any potential charges would have been around her knowledge of Vincent receiving 120,000 pounds in match-fixing bribes.
Mr Pownall asked the jury to disregard the Crown's comparison of Cairns to Lance Armstrong, as that was irrelevant.
He pointed out Cairns was never caught in a lie despite feisty exchanges when he was giving evidence in the witness stand.
"He is vulnerable like anyone else. He was vulnerable, but he wasn't shown to be a liar," he said.
"That's not because he's some clever dick who is able to evade, he's a combative individual."
Jonathan Laidlaw, QC, will give a closing speech on behalf of Andrew Fitch-Holland tonight (NZ time).
Mr Pownall asked the jury to consider why Cairns would seek to recruit McCullum to cheat, as he was a young player yet to reach the "zenith of his career" paid large sums of money in the Indian Premier League, not a "flakey individual" who would risk his career for the sake of a "few extra dollars".
This was a big risk for Cairns to take, said Mr Pownall, as McCullum would be likely to reject him and report him to the authorities.
He also questioned why Cairns, also earning a significant salary in the Indian Cricket League, would recruit just one player in the rival IPL when several corrupt players are needed to "turn a game".
