A rare multimillion-dollar violin, reputedly made by Stradivarius and once owned by Liberace.

The aging mob hit man who hid it in the attic of his vacation home before he was locked up and placed under the tightest security the U.S prison system can muster.

And a corrupt prison chaplain, who allegedly broke the law to try to help the mobster recover the valuable instrument before authorities could find it.

It might sound like a plot rejected by “The Sopranos.”

But every so often Chicago’s federal courthouse serves up a real life drama so unlikely that even the most shameless Hollywood scriptwriter would cringe at its contrivance.

And on Wednesday, the long-awaited trial of 66-year-old Catholic priest Eugene Klein promises to be just such a show.

GUILTY PLEA: Priest faces up to five years for helping mob killer

Prosecutors allege Klein — the former prison chaplain at the Federal Penitentiary in Springfield, Mo., — acted as a secret messenger for feared and prolific killer Frank Calabrese Sr., delivering notes the mob boss had written in solitary confinement, then hidden inside religious books, to an associate on the outside.

Together with that associate, Klein plotted to snatch back the violin and sell it, the feds allege. They say Klein “confessed” and gave them a note Calabrese had passed him through the slot in his prison cell door, describing how to find the hidden violin at his Wisconsin vacation home.

“Be sure to have a little flashlight with you so you can see,” the note said. “Make a right when you go into that little pull-out door. Go all the way to the wall. That is where the violin is.”

Klein doesn’t dispute that he passed on the messages for Calabrese, who died in prison in 2012. But his attorney, Thomas Durkin, has ridiculed the case. He questions whether the violin, which has never been recovered, ever existed in the first place. And he hopes to convince U.S. District Judge John Darrah at what’s expected to be a four-day bench trial that Klein didn’t conspire to violate the strict prison rules under which Calabrese was being held.

Convicted at the landmark 2007 “Family Secrets” mob trial, Calabrese, then 70, was sentenced to life behind bars and ordered to pay $4 million in restitution to the families of his victims by a judge who said he was responsible for 13 mob hits. The government seized his homes in Oak Brook and Williams Bay, Wis., and held him under “special administrative measures,” an extreme form of solitary confinement typically reserved for the most dangerous terrorists and organized crime figures.

His contact with the outside world was limited to legal discussions with his attorney, closely monitored meetings with a select group of close relatives, and Klein.

As the prison chaplain, the feds say, Klein knew the rules. But in March 2011 he was caught on a security camera pocketing what he said was a candy bar that Calabrese had passed him through the slot in his cell door. Challenged by the feds, he “confessed,” shared the note about the violin and said he had met with an associate of Calabrese’s at a restaurant called “Zsa Zsa’s” in Barrington, the feds say.

At that meeting, Klein told the associate that Calabrese had told him the violin was a Stradivarius worth millions that at one time had belonged to Liberace, that Liberace’s lover had sold the violin, and that “somehow Calabrese had ended up with it,” the feds allege.

Though they offered no explanation for how Calabrese got the violin, he was known to take things of value from people who owed him money.

But Klein’s alleged plot to recover the violin was challenging because the vacation home was being sold to pay Calabrese’s victims, the feds say. Klein and the associate allegedly planned to pose as potential buyers so that they could take a “tour” of the property, then distract a realtor for long enough to grab the violin and run.

When the house sold before they could carry out the plot, he considered trying to actually buy the house himself, the feds say. The pair allegedly believed the violin was worth as much as $26 million, some of which Klein planned to use to hire an attorney for Calabrese, and some of which he allegedly intended to keep for himself.

Durkin, though, this week dismissed the $26 million valuation as something Calabrese’s associate had seen on television.  And he wrote in a court filing that “the very existence” of the violin “is anyone’s guess.”

A search by federal agents of Calabrese’s Oak Brook home in 2010, however, did uncover a secret basement compartment filled with cash and jewelry worth more than $1 million, and a certificate for a 1764 violin, described as a “Stradivari” but made by the lesser Giuseppe Antonio Artalli, not Stradivarius.

Klein, who is in poor health and walks with a stick, buried his mother last week. He faces up to 10 years in prison, if convicted.