TALLAHASSEE, Fla. — With a plot that echoes a well-known Hollywood masterpiece, a trial began Tuesday for a in order to secure a massive insurance payout. Prosecutors contended that planned the December 2000 slaying of her husband with a man that she was having an affair with and later married.

On a cold morning in north Florida, Mike Williams disappeared while duck hunting on a large lake west of Tallahassee, and initially it was believed that he had fallen from his boat and that his body had been devoured by alligators. His disappearance triggered a massive search by local and state authorities.

As "48 Hours" correspondent Richard Schlesinger reported, Williams' mother, Cheryl Williams, never believed her son was eaten by alligators. She compiled 27 pages of single-spaced notes and evidence and wrote the governor of Florida every day for nine years.

It was revealed years later that Williams had died from a shotgun blast to the head and had been buried near a lake north of the state capital. The man who shot him was his best friend and insurance agent Brian Winchester, who confessed last year to the killing and repeated that confession in court.

Mike and Denise Williams Russell Grace

"Denise really didn't have to do a whole lot other than come up with an alibi for herself and make sure that Mike went," Winchester said in court on Tuesday, CBS affiliate WCTV reports.

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At the time of his death Williams had three life insurance policies worth $1.75 million.

"Sex, lies and deceit," was how Assistant State Attorney Jon Fuchs described the case against 48-year-old Denise Williams, which resembles the plot of the famed 1940s film noir classic "Double Indemnity."

Williams is charged with first-degree murder, conspiracy to commit murder and accessory after the fact in her husband's killing.

Without a body, Denise Williams petitioned to have him declared dead due to accidental drowning. She married Winchester in December 2005, but the relationship soured and they divorced in 2016.

Last year, Winchester pleaded no contest to kidnapping his ex-wife at gunpoint, and was sentenced to 20 years in prison. The very next day, authorities announced that Mike Williams' remains were found at the end of a dead-end road after they received "new information." The body had been discovered two months earlier, but the state's Department of Law Enforcement needed to confirm through DNA tests that it was actually Mike Williams.

Philip Padovano, one of the attorneys representing Denise Williams, asserted to jurors on Tuesday that there is no proof his client was having an affair with Winchester or had helped plot the killing.

"There's no tangible evidence or physical evidence to connect Denise Williams to this crime," Padovano said. "No confession, no admission, nothing."

He told them that Winchester has been given immunity from murder charges in exchange for his testimony and that he was given a lighter sentence in his kidnapping case. In order to find Williams guilty, Padovano told the jury that "you will have to rely on the word of a murderer and a convicted felon."

In testimony later Tuesday, Winchester described his long-running affair with Denise Williams. He said Mike Williams told him that Denise had stopped having sex with him and that he was growing suspicious about her activities. Winchester said the growing suspicion and the fact that one of Mike Williams' insurance policies was about to lapse led him and Denise to kill him.

"We knew our window of opportunity was closing," Winchester testified.

He then tearfully described the plot to kill Williams. He said he had intended to drown him by pushing him from his canoe. But he said he panicked when he saw Williams struggling in the water to get out of his jacket and duck waders and shot him. He said he dragged Williams' body back to shore, put him in his truck and buried him hours later.

"When I shot him, it was dark and there was a bright flash," Winchester testified, adding later, "I had to make it happen, I had no choice."

The first day of the trial ended before attorneys for Williams had a chance to cross-examine him.