This plot was too crazy even for Charles Manson.

Manson’s engagement to a woman 53 years his junior was part of a wild scheme of hers to profit by putting his body on public display after his death, says the author of an upcoming book.

Manson’s fiancée, 27-year-old Afton Elaine Burton, known as Star, sought to wed the convicted mastermind of the Sharon Tate murder and eight other slayings so that she could gain possession of his corpse, according to journalist Daniel Simone.

Burton and a pal, Craig Hammond, planned to lay out Manson’s remains in a glass crypt, Simone says. The pair figured their bizarre California version of Lenin’s Tomb would draw huge crowds and make big money.

But Manson, 80, does not want to marry Burton and has no interest in spending eternity displayed in a glass coffin, Simone told The Post. “He’s finally realized that he’s been played for a fool,” Simone said.

Another reason the madman balked at the plan is because he believes he is immortal. “He feels he will never die,” Simone said. “Therefore, he feels it’s a stupid idea to begin with.”

Manson’s and Burton’s marriage license expired Thursday. “They plan on renewing the license, and things will move forward in the coming months,” says a statement posted on Burton’s and Hammond’s website.

Burton and Hammond — who uses the nickname Gray Wolf — could not be reached for comment.

The wedding was postponed “due to an unexpected interruption in logistics,” the site says. Manson entered a prison medical facility for treatment of an infection about two months ago and cannot receive visitors, Simone said.

California prison officials would not comment on either Manson’s medical condition or his whereabouts.

Simone and a collaborator, Heidi Jordan Ley, are seeking a publisher for their book, “The Retrial of Charles Manson.” Manson and other inmates at Corcoran State Prison in California are helping the project. The authors say they spoke with Manson regularly before his phone privileges were suspended two years ago. They also corresponded with Manson and his inmate friends by mail.

According to the authors, Burton and Hammond hatched their plan to display Manson’s corpse about two years ago.

Initially, the duo asked Manson to sign a document that would let them take his body when he dies.

“He didn’t give them a yes, he didn’t give them a no. He sort of strung them along,” Simone said. Burton and Hammond regularly brought Manson toiletries and other items. Stringing them along kept the goodies coming, Simone said.

When it became clear last year that the purportedly death-proof Manson would not green-light the plan, Burton and Hammond switched to the marriage idea, Simone says.

If Burton married Manson, the pair realized, California law would give her possession of Manson’s remains upon his death, Simone said.

Simone believes Manson never intended to marry Burton. “Manson never consented to the wedding in the first place and never will,” he said.

Another Simone book, “The Lufthansa Heist,” about a $6 million armed robbery at Kennedy Airport in 1978, is due out next month. He co-authored it with “Good­Fellas” mobster Henry Hill, who died in 2012.