New York- A handyman working at David Letterman’s Montana ranch hatched a sick plot to kidnap his baby boy for a $5 million ransom – but was caught after he blabbed about the plan to a pal, authorities revealed yesterday.

Prosecutors said Kelly Frank’s scheme against the late-night comedian was no joke: He knew where 16-month-old Harry Letterman’s crib was kept, took pictures of the property and procured a key to the house.

He even figured out a way to keep the adorable tyke fed and in clean diapers while shaking down his famous father.

“Part of the plan was to kidnap the nanny so that she could take care of the child until they were returned or set free,” prosecutors wrote in an affidavit.

Letterman, who did not tape his CBS show yesterday, had no comment on the shocking charges except to thank the FBI and police for their “tireless efforts and determination.”

Frank, 43, who’s been on probation since a 1999 conviction for intimidation and stalking, had a job painting buildings on Letterman’s 2,700-acre ranch in Choteau, Mont.

Sometime in the last few weeks, he confided to a close buddy, Robert Gondeiro, who also works at the ranch, that he had a get-rich-quick plan.

He allegedly told Gondeiro he knew Letterman and his son would be arriving at the ranch soon and that it would be a cinch to snatch the little boy and his nanny, leave a ransom note and hold them captive for 48 hours.

“He believed Letterman would pay $5 million for the safe return of the child,” prosecutors charged.

Frank wanted his friend to help him with the kidnapping – but Gondeiro went to cops on Sunday night and spilled the beans instead, authorities said.

Montana cops called the FBI, and they hauled in Frank for questioning.

He eventually confessed to everything – and even admitted he overcharged Letterman for the paint job, the affidavit said.

His father, Robert Frank, 75, told the Daily News he hoped the authorities had made some kind of mistake.

“I didn’t see this coming,” he said. “I just can’t imagine him even thinking like that. I don’t think he’d ever seen Letterman in his life.”

When he learned it was Gondeiro who had betrayed his son, the elder Frank was stunned.

“That’s his best friend. I’ll be damned!” he said.

Wearing striped prison overalls and a droopy mustache, Kelly Frank appeared before a Teton County justice of the peace yesterday to be arraigned on a felony solicitation charge for the kidnapping plot.

He was also hit with a felony theft charge for the paint job ripoff and an obstruction charge for initially lying to the FBI agent.

Bail was set at $500,000, though Justice John Howard tacked on an extra $100,000 because Frank allegedly violated probation.

The earlier case against Frank apparently stemmed from a domestic dispute; he was initially charged with rape and kidnapping but pleaded guilty to lesser charges.

Frank, who has a live-in girlfriend and two children, barely spoke at the hearing, which was attended by three representatives for Letterman.

Letterman was believed to be out of town last night, but it’s unclear whether he was at the Montana ranch.

At the Ed Sullivan Theater, where his show usually tapes, fans were aghast at the news.

“That is horrible, just absolutely disgusting,” said Sara Labance, an environmental engineer from Vernon, N.J., who was trying to get tickets for “The Late Show.”

“He seems like one of the most devoted fathers. Besides being funny, I have the utmost respect for him. I think it’s unconscionable someone would even contemplate such a heinous crime.”

Additional: Notoriously cantankerous David Letterman has made a career out of prickly interviews, but one topic of conversation is guaranteed to soften him up – his son, Harry Joseph.

Letterman always has been intensely private, but becoming a first-time father late in life seemingly has opened him up emotionally on-air.

“I could never imagine ever being a part of something that turned out this beautiful,” Letterman said on his first show after Harry’s Nov. 3, 2003, birth. “It was overwhelming, when they handed him to me I just exploded and couldn’t stop crying.”

When he interviewed Howard Stern during that telecast, Letterman candidly admitted that “when [Harry showed up], everything changed.”

“My father passed away when he was 57,” Letterman said on that show. “I’m 56 years old, and yesterday I had my first child. So I named him for my father, and his name is Harry Joseph Letterman. So God bless Dad and God bless Harry.”

Surprisingly serious sentiments aside, the child also has opened up a whole new slew of comedic material.

“First thing, I took him home and dangled him over the balcony,” Letterman joked, referring to Michael Jackson’s infamous stunt. One of his “Top 10” lists detailed the good things about being a father, with No. 1: “There is now tangible evidence that I have had sex.”

Many of the guests of the “Late Show” have come to the Ed Sullivan Theater bearing gifts for the little Letterman – including President Bill Clinton, who brought a copy of his 957-page book “My Life” for the then-9-month-old – and the child remains a favorite topic of his father’s monologues.

“There’s so many things I want to teach the kid,” Letterman joked recently. “I can teach him fishing, how to lay down a bunt … how to fake his way through an interview with Angelina Jolie.”

Additional: Before the hired help allegedly started plotting against him, the late-night host had been harassed by a suicidal stalker, a wild black bear and, of course, those pesky traffic cops.

Trouble sticks to David Letterman like a Velcro suit, and for a pretty private guy, he’s been in the headlines an awful lot over the years.

The most notorious incidents involved Margaret Mary Ray, the schizophrenic who latched onto him in 1988 and began breaking into his home.

She stole his car, claimed she was his wife, slept on his tennis court – and did time in prison and mental hospitals before killing herself by kneeling in front of a train in 1998.

The Ray case was tragic, but some of Letterman’s other problems have become fodder for his monologues – such as the speeding tickets he’s gotten from Connecticut police.

In 2003, Letterman joked with his audience about a 350-pound black bear with a bad case of the munchies that broke into his Montana ranch and ransacked the fridge.

The bear returned to the house two more times before state wildlife officials finally relocated it to the other side of the Continental Divide.

Letterman also has found himself in the spotlight because of some health problems: in 2000, he had open-heart surgery and three years later, he was off the air for five weeks with shingles near his right eye.