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 them 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

Since you asked, I'll share another one! This one's from TBC's 1976 daily drama "Lady of the Side Chamber," an adaptation of the Joseon novel "Legend of Lady Park". The show itself was Season 6 of a 14 part anthology on famous women real and fictional from the Joseon Dynasty. Only two episodes of this season remain.

Trailer

Plot:

Deep in Mt Geumkang there lived a Taoist Master named Master Yujeom and his daughter Lady Park. Yujeom was best friends with Court official Lee Deuk Chun, and they swore to marry their children together. One day, Deuk Chun takes his son Shi Baek to Yujeom's house, but Shi Baek nearly dies on the mountain. He's saved by a mysterious woman in a black veil, and falls in love with her immediately. Upon reaching Yujeom's house, Shi Baek finds out the veiled woman is his bride to be, but becomes afraid of her veil that she can never take off (it's because she's really ugly), and won't enter the bridal chamber with her.

After marrying, Shi Baek, Deuk Chun return to their mansion, named the Chwihee Hall, and her new in-laws are frightened by how ugly Lady Park is, so they avoid her. Lee Shi Baek distances himself from his wife and stays with a courtesan named Sanwol, whom he loved since before getting married. Sanwol lives in a tavern with her mother Kang Shim. Shi Baek's mother Lady Kang and his aunt pressure Lady Park to get divorced and go back to her family. Only Lee Shi Hwa, Shi Baek's sister, befriends Lady Park and frequents Chwihee Hall.

As time goes along, Shi Baek treats Lady Park even more coldly, while the other in-laws scorn and bully her even more, to the point that she begs them to build a side chamber where nobody can see her. They name it the Pihwa Pavilion, a really insulting name as it means "avoiding disaster". Lee Shi Hwa suggests wearing a white veil instead of a black one, and Lady Park agrees. Lady Kang, Sanwol and Sanwol's mother Kang Shim are eager to chase her out, and scheme to get Sanwol into Chwihee Hall as Shi Baek's concubine. Kang Shim finds an extraordinary fortuneteller and curses Lady Park, but Lady Park uses her powers of endurance to overcome it.

One day, Lady Park thinks of a way to make money for her father-in-law and spends $300 on a sick horse in the marketplace that's worth $7. She asks her father-in-law to feed it everyday with sesame seed rice porridge. Aware of her abilities, Lee Deuk Chun does as she asks, and after three years, it becomes an excellent horse with supple skin and sheeny hair. She sells the horse to a Ming emissary for $38,000. As Lady Park expected, the money from the horse startles the Lee family.

Three years into their marriage, Lee Shi Baek's civil examination approaches and Lady Park has an unusual dream, where her husband gets first place after touching a special set of brushes, paper and inkstone that she sent him. The theme of the civil exam was denoted in the shadow of the inkstone. Lee Shi Baek is inspired and regrets his actions towards her. He passes the civil exam, but finds out Lady Park promised his mother to kill herself if he passes. Fortunately, Lady Park's father Master Yujeom visits and uses magic to fix her face, which is now as beautiful as a fairy's. Lady Park's marital chemistry with Lee Shi Baek has improved, but Lady Kang's fear changes to jealousy, and continues to find fault with her. Even so, Lady Park treats Lady Kang with sincerity. When she gets pregnant, Lady Kang is moved and begins to treat Lady Park with love. From then on, the family begins to get along. Lady Park gives birth to twins, now served by new servants Gyehwa and Gil Bo. Lee Shi Hwa also marries her fiance Ju Il, to the joy of the entire family.

Meanwhile to the North, the Manchu nation of Later Jin is growing more powerful, and the clouds of war begin to hang. Lee Shi Baek, who's just come back from Later Jin as an emissary, prepares for an attack and petitions the Court to reinforce national defense, but the Court officials oppose and King Injo doesn't believe Later Jin will attack. Lady Park is frustrated and uses her supernatural powers to appear in the King's dreams. She speaks to him of the situation and lets him know that Later Jin will change their name to Qing and attack. She suggests that he think of a countermeasure. Soon after Injo awakens he summons the Court officials and asks Lee Shi Baek in detail about what he saw in Later Jin, but the Court officials still oppose.

Some time later, a beautiful courtesan named Seoljungmae appears at Kang Shim's winehouse and entrances Lee Shi Baek. Lady Park is anxious over these news and invites Seoljungmae to meet her herself. She finds out that Seoljungmae is secretly a Qing Princess here to assassinate Lee Shi Baek, and then scolds Seoljungmae before sending her back. At the same time, she tells Injo that the Qing troops will be here soon, so he should urgently avoid disaster. The King sentences her to capital punishment, and the Court requests in unison to kill both her and Lee Shi Baek.

Over in Qing, Hong Taiji leads Tatara Inguldai (Long Gu Da/Yong Gol Dae) and his brother Long Hu Da/Yong Hol Dae to begin their attack on Joseon. As the Qing army of 100,000 approaches, Injo escapes to Namhan Mountain Fortress. Inguldai chases Injo to the fortress while Long Hu Da attacks and plunders the people. Long Hu Da's soldiers break into the side chamber at Pihwa Pavilion but Lady Park slits their throats with her magic and locks the door.

King Injo finds out Namhan Fortress only has provisions for 50 days, and regrets not listening to Lee Shi Baek. A heated debate arises when he suggests a surrender, saying Lady Park represents Heaven's will and that they must surrender to Hong Taiji. He writes a correspondence and takes the Crown Prince with him in common clothes to go kneel before Qing.

After the Manchu Invasions, Lee Deuk Chun becomes Chief State Councilor, Lee Shi Baek becomes the Crown Prince's mentor, and Lady Park becomes Lady Jeonggyeong (First Class Noblelady, highest rank a non-Royal woman can attain) for repelling Inguldai and Long Hu Da with her magic. The family gets a Gate of Loyalty and a Pavilion tiled with Palace tiles.

Master Yujeom's dying day approaches and so he praises all of his daughter's achievements, visiting one more time to see her twins. Master Yujeom looks at their faces and predicts that they and their children will become State Councilors one day. Then, he chooses his day to die. Lady Park is extremely sad.

The King joyfully spends his time in the new Pavilion, calling Lady Park and Gyehwa to compliment them. 14 years later, King Injo dies and his son Hyojong comes to the Throne. Hyojong, who spent years as a hostage in Qing after Injo's surrender, is resentful against the new Chinese dynasty and plans the Northern Conquest, a military plan to conquer the Qing Dynasty. Lee Shi Baek is now Chief State Councilor, busy with State affairs. He and Lady Park have sown resentment in their hearts and are impatient to one day conquer the North, but both know their time is coming soon. They both find a corner of their mansion and spend their last moments together, leaving behind one words as advice to the family: Loyalty [to the nation].

Historically the Northern Conquest was doomed at inception because Joseon simply had no money or resources to prepare for something like that after 4 invasions in 60 years. Hyojong died without realizing his plan, and his son Hyeonjong disbanded it upon ascension.