Hm, not the longest but definitely the weirdest - this one's a daily historical called Oknyeo (1978) from channel MBC (who's still around today), a remake of a 1970 weekly drama of the same name from channel TBC. TBC was the biggest SK terrestrial channel of the 70s, abruptly ended when Jeon Du Hwan's military dictatorship shut them down in 1980 and incorporated them into government channel KBS. TBC's corpse remains today as KBS2, while a cable channel jTBC started in 2011 as a resurrection by the original channel owners, the Joongang Group.

The original plot was:

Lee Gil Jae (Geungjae in some episodes, probably a mix-up with the male love interest from competing drama My Lady), a nobleman, loves Heo Oknyeo, a fallen noblewoman from a different political faction, but his parents arrange his marriage to Yihwa, a noblewoman from their political faction. Gil Jae tries to run off with Oknyeo, but they are caught by Kim Yun (don't know his relation to anyone). Unable to do anything, Gil Jae marries Yihwa in despair, and promises to build a future with Oknyeo one day. Oknyeo becomes Yihwa's servant, and is greatly favored by the sickly Yihwa. Gil Jae has secret meetings with Oknyeo, which lead to a murder attempt by a certain Scholar Song (don't know his relation to anyone). Unaware of the whole situation, Yihwa misunderstands and believes Oknyeo is having an affair with another scholar Park (don't know his relation to anyone). Oknyeo gets pregnant with Gil Jae's child, making Yihwa extremely jealous, and Oknyeo's father Secretary Heo, formerly presumed dead, reappears as a beggar. Court strife comes between Oknyeo and Gil Jae, as their families are from different factions, and they are forever unable to attain happiness. Oknyeo dies at the end.

Pretty tame. But then 8 years later MBC extended the plot to (I'm including the marketing catchphrases and description from the newspaper ads because wow tv marketing sure has changed)

"Mother, what do we do about Oknyeo? The Young Master's love moves one to tears. His father hates her to death. But there's nothing Oknyeo can do!" "The tragic story of one woman of the Lee Dynasty, wandering in search of her lover, determined by a fate that transcends time and space!"

Plot:

During Prince Gwanghae's reign in Joseon Korea (1608-1623), Yun Sang Heon's only daughter Oknyeo dies of illness. During her funeral, Oknyeo climbs out of her coffin to everyone's surprise. Her prospective fiancee Kim Shijin backs out of their future marriage after this, and all chances of her getting married are broken after he finds out Oknyeo can now predict the future by looking into her mirror. Rumors spread to Kim Gae Shi (Gwanghae's closest confidante and the most powerful Court lady in the Palace, more powerful than the Queen) who invites her into the Palace to act as her fortuneteller. Oknyeo becomes famous and acts as Gae Shi's helper for a while. One day, Oknyeo sees the ghost of her mother Lady An, who predicts that her true love is ex-suitor Lee Se Young, son of Lee Myeong Ha. Seyoung is set to marry a maiden named In Shil, but he's still very much in love with Oknyeo, and confirms it before the official engagement arrives.

Suddenly, Oknyeo and her father are caught in conspiracy, and by Kim Gae Shi and Lee Myeong Ha's schemes, Yun Sang Heon is ordered poisoned. Oknyeo too, but she manages to escape punishment. This enrages Lee Myeong Ha, who lies to Kim Gae Shi and tells her they're both dead. Oknyeo runs to Mount Dobong, Lee Myeong Ha at her heels. She dresses as a man and hides out at the house of a woman named Suwon. Oknyeo uses her shamanic powers to exorcise Suwon's husband, who is haunted by the ghost of her sister-in-law, but Suwon sells her out to Lee Myeong Ha anyway. When the officers arrive, Oknyeo has already disappeared.

Seyoung and Inshil are about to be married, but Seyoung gets a shock when Inshil's maid Gyehwa turns out to be Oknyeo. Seyoung loses his mind for like a week, then gets locked up because everyone thinks he's faking. Oknyeo begs Inshil to be Seyoung's concubine. Inshil gets married to Seyoung, to Oknyeo's frustration. She looks to her mirror again, wondering if Seyoung and her are truly destined to be with each other. However Seyoung leaves the bridal chamber after the wedding and goes straight to Oknyeo, getting her pregnant. Oknyeo then predicts Prince Neungchang (Prince Gwanghae's distant relative who is a threat to his legitimacy as King)'s death, which starts more chaos. Somehow Seyoung gets himself involved and Inshil ends up protesting outside her own house on a mat, refusing to eat. Queen Yu (Prince Gwanghae's wife), who's been working with Kim Gae Shi the whole time, reveals Oknyeo's identity.

Meanwhile in Court, Court officials Kim Yu and Jeong In Hong try to kill Lee Myeong Ha, who is still out for blood. Oknyeo hides in Jeong In Hong's house and tries to kill herself, but her suicide attempt is foiled when she realizes Kim Yu switched out her poison for baby medicine. In Hong dreams of Lee Myeong Ha killing Oknyeo with poison, but Oknyeo ends up trying to poison someone named Song Sa Cheon because he found out who she really is. Song Sa Cheon reports to Kim Gae Shi, and pressures Lee Myeong Ha to prove if Oknyeo can really see the future.

Now everyone's on the run for Oknyeo, including Lee Myeong Ha, Queen Yu, Kim Gae Shi and Song Sa Cheon. Oknyeo hides with Suwon again and everyone's about to find her when.... the issue of deposing Queen Inmok suddenly comes up and everyone in Court gets distracted. Kim Gae Shi starts the chase again when she needs to know if she'll be successful in deposing Queen Inmok, and with nowhere left to run, all chances of leaving the Capital dashed due to her imminent delivery, Oknyeo gives birth at the same time Lee Myeong Ha coincidentally dies, and then kills herself. Queen Inmok is then deposed (but that's its own story)

Edit: come to think of it there's one more I think is crazier than this but it's based on a novel