The mistress nanny who testified against her former flame, ex-IRS agent Brendan Banfield, was lying when she told jurors he killed his wife in an elaborate, kink-heavy frame job plot, the defense argued before deliberations began Friday.

The jurors in a court in Fairfax, Virginia, now have to decide whether Brendan Banfield stabbed his wife, Christine, and shot a stranger, Joe Ryan, on whom he planned to pin the slaying.

But first, the panelists heard closing arguments from Banfield’s lawyer, John Carroll, who told them that the testimony of the Brazilian au pair, Juliana Peres Magalhães, couldn’t be trusted and that cops had their minds made up from the start — that the murders were part of a “catfish” scheme.

“Juliana made it up,” Carroll said. “She told the commonwealth what they wanted to hear.”

“She told this lie to secure herself a deal that benefited her,” the defense attorney argued. “She was a pawn in trying to get to my client.”

“Her entire story has been bought and paid for,” Carroll said — the payment being her freedom and the opportunity to sell her story for a documentary.

Carroll also said Banfield didn’t shy away from admitting the bad things he did when he testified to jurors Thursday, including admitting having multiple affairs — all of which shows he was telling the truth when he claimed he wasn’t to blame for the double murder.

“My client doesn’t have to testify.” Carroll said. “Yet he gets up there and says he’s a philanderer. He had affairs … he told you, ladies and gentlemen, exactly what was going on and exactly how it went.”

Meanwhile, prosecutor Jenna Sands said her office had all the “uncontradicted evidence” it needed to prove Banfield’s guilt — even without Peres Magalhães’ damning testimony.

Sands also told jurors not to buy Banfield’s claims on the stand that the Brazilian au pair was the driving force behind the affair and that it was some casual dalliance.

“He was in love with Juliana,” Sands said. “He can pretend this was a fling … He was afraid of losing her. He needed to get rid of his wife so they could be together, so they could get married, so they could have children together.”

Sands claimed that Banfield, 40, a former special agent with the federal tax agency, carried out the slayings mere months after starting the tryst with the nanny of the couple’s only child.

And the cheating husband planned an elaborate plot in which he opened an account on a fetish site, posing as Christine, and arranged for Ryan to help his wife play out her supposed desire for a rape fantasy, prosecutors said.

So on Feb. 24, 2023, Ryan came to the Banfields’ home in Herndon, Virginia, armed with a knife to carry out the depraved sex session with Christine — a former sexual assault nurse who believed she was actually being raped.

The nanny, sitting in a car outside the home with the Banfields’ 4-year-old daughter, called Banfield, telling him of an intruder to set up their narrative with evidence. So Banfield rushed home and shot Ryan with his service weapon to make it appear as though he’d valiantly saved his wife, Sands said.

Banfield then stabbed Christine in the neck multiple times — all while their little girl was left unattended in the basement, Sands said.

Banfield has maintained his innocence. 

Jurors are tasked with deciding three charges: aggravated murder, a weapons charge tied to the murder, and endangering his daughter.

The nanny pleaded guilty and is expected to be let off with a sentence of time served as part of a deal with prosecutors for her cooperation and testimony. She is slated to be sentenced sometime after Banfield’s trial concludes.