The romance between two Hollywood legends, Frank Sinatra and Marilyn Monroe, was a long-kept secret that continued for seven years.

The lovebirds started as friends and eventually became lovers, with Monroe calling Sinatra “Frankie” and Sinatra calling her “Norma Jeane”.

Their love story began in 1953, when both stars were at the peak of their careers.

They found that they shared a lot in common, including the pressure of public scrutiny and insecurities due to their fame.

Sinatra liked to save women in distress, and Monroe turned to him during tough times.

Both were vulnerable and tormented by loneliness and isolation.

Monroe’s troubled past included abandonment by her father and a schizophrenic mother whom she had hidden in a mental institution for years.

She also had a tumultuous relationship with her ex-husband, baseball legend Joe DiMaggio, who watched her every move like a stalker.

Sinatra, on the other hand, had hit rock bottom in his music career before making a comeback with his Oscar-winning performance in “From Here To Eternity”.

Their romance might have gone public if joint plans for a movie and concert tour had not faltered.

Monroe and Sinatra were set to star on-screen in the musical comedy “The Girl In Pink Tights,” but Monroe dropped out when she learnt she would be paid less than a third of Sinatra’s salary.

Plans fell through to co-star in the movie “Guys And Dolls,” and a remake of “A Tree Grows In Brooklyn.”

Friends had warned Monroe to stay away from Sinatra, but she didn’t listen.

Bandleader Tommy Dorsey cautioned her, “Frank is the most fascinating man in the world, but don’t stick your hand in the cage.”

Sinatra was hesitant to tie the knot with Monroe, knowing that her ex-husband, Joe DiMaggio, was a head taller than him and had mafia connections.

The couple even planned to marry, but both knew how jealous DiMaggio could be, and so they kept their relationship a secret.

“Frank and Marilyn wouldn’t dare go public, not even holding hands on a red carpet,” says Epstein.

Their affair was known only to their closest Hollywood confidantes.

They found excitement in the cloak-and-dagger secrecy of their romance.

They walked a perilous tightrope, balancing between Monroe’s affairs with President Kennedy, his brother Robert, and her possessive ex-husband, Joe DiMaggio.

Monroe and Sinatra realized that they wanted to pursue other relationships while remaining lovers.

They enjoyed s^x without any reservations, and Monroe was at the apex of a love pentangle with Sinatra, both Kennedy brothers, and DiMaggio.

“Marilyn had passion for all four men,” says the author.

Even during their relationship, Sinatra and Monroe indulged other lovers.

Monroe bedded Marlon Brando and married playwright Arthur Miller.

Sinatra romanced Gloria Vanderbilt and was briefly engaged to Humphrey Bogart’s widow Lauren Bacall.

John and Robert Kennedy dangerously added their libidos to the mix.

“It was Frank who introduced Marilyn into the Kennedys’ circle, not anticipating them becoming her lovers,” says Epstein.

“Marilyn honestly thought she would marry JFK and he’d divorce his wife Jackie.

“But when the Kennedys dumped Marilyn and stopped taking her calls, she threatened to go public about their affairs.”

Her fling with Robert Kennedy had sent Monroe into a downward spiral, teetering on the verge of a nervous breakdown.

Sinatra invited her to join him to recuperate at his Cal-Neva resort hotel on the California-Nevada border.

However, an insecure Monroe begged DiMaggio to stay in a nearby hotel in case she needed him.

At Cal-Neva, Monroe overdosed on barbiturates, and Sinatra walked her around the room battling to keep her awake.

“She wants to kill herself,” he told friends.

“I’ve been there.”

Says Epstein: “Marilyn fell apart at the very end. She didn’t have the strength to, in Frank’s words, pick herself up and get back in the race.”

Shortly after returning to Los Angeles, Monroe was found dead at the home of a drug overdose.

Before going to bed for the last time, she had played Sinatra records.

Sinatra himself died in 1998 at the age of 82.

The legendary singer and actor had been married four times and had a long and storied career in show business.

The relationship between Sinatra and Monroe remains one of Hollywood’s most intriguing and enduring mysteries.

Their affair, while not publicly acknowledged until years after their deaths, has become the stuff of legend.

As Epstein notes in his book, “Frank & Marilyn,” the couple’s relationship was one of “desire and dysfunction.” They were two of the biggest stars of their time, both struggling with their own demons and insecurities.

While their love affair was ultimately doomed, it has remained a source of fascination for fans and historians alike.

The fact that their relationship was able to remain a secret for so long is a testament to the power of Hollywood’s star system and the allure of two of its biggest icons.

As Epstein writes, “Frank and Marilyn were Hollywood’s best-kept secret.”