The film begins with Rahul Khanna (Shah Rukh Khan) standing in front of the funeral pyre of his wife Tina (Rani Mukerji). Through a series of flashbacks we are shown Rahul and Tina's marriage, their honeymoon, Tina's pregnancy with their daughter and the child's eventual birth. It is revealed that Tina was aware of complications in her pregnancy, during which only mother or child could be saved, but decided not to tell Rahul as she knew how much he wanted a child. During her pregnancy, Tina, who also loved the baby more than her own life, writes eight letters to her daughter (one for each of her first eight birthdays) and gives them to Rahul's mother (Farida Jalal). Before dying, Tina asks Rahul to promise her to name their daughter Anjali.

Eight years go by and Rahul is a single parent living with his daughter Anjali Khanna (Sana Saeed) and his mother. His mother worries that Rahul is lonely and that Anjali is missing out on a motherly figure in her life, but Rahul brushes it off saying that having him and his mother is enough. On her eighth birthday, Anjali reads the last and most important letter that her mother left her; it tells the story of Rahul, Tina and Anjali.

A flashback begins from St. Xavier's College, where Rahul Khanna and his best friend, tomboy Anjali Sharma (Kajol) are popular students. The two are close friends but deny any romantic feelings for each other. The college principal Mr. Malhotra (Anupam Kher) states that his daughter Tina is coming to St. Xavier's from London to finish her degree, and asks Anjali to be her guide and friend. Tina quickly becomes friends with Anjali and Rahul, and Rahul constantly flirts with Tina, trying to win her over. As time goes by, Anjali realises that she is in love with Rahul, and is encouraged by her motherly guardian Rifat Bi (Himani Shivpuri) to tell him. Before she is able to declare her feelings for him, Rahul instead tells Anjali that he is in love with Tina. Devastated, Anjali decides to leave college and shares a tearful goodbye with Rahul. Although she is in love with Rahul, Tina realises that she came between him and Anjali.

Back in the present, Tina explains in her letter to her daughter Anjali Khanna that Rahul always said that love is friendship, and without friendship there can be no love. Rahul is a single parent and lacks a friend in his life, so Tina gives her daughter the mission of reuniting Rahul with Anjali and bringing back Rahul's lost love and friend. The next day, Rahul, Anjali Khanna and his mom head to Faridabad where they meet Mr. Malhotra for Tina's death anniversary, where Anjali shows them, except for Rahul, her mother's letters..

Anjali Sharma is no longer the tomboy she was in college, she has become a beautiful woman and is now engaged to Aman Mehra (Salman Khan), an NRI living in London. However, she explains to her mother (Reema Lagoo) that she does not really love him and is marrying him as a compromise as she feels she can never love again after losing Rahul. Aman also suspects that she does not really love him. Determined to reunite her father and his best friend, Anjali Khanna and her grandmother search for Anjali Sharma and soon learn that she is going to be working at a summer camp in Shimla. Rahul tells his daughter that she is not allowed to attend the summer camp and heads to a conference, only to find out that his mom and daughter went without telling him, having left a letter for him. They travel to the camp, with a plan to later lure Rahul to come as well, and the two Anjalis meet each other. One night while watching television, Anjali Sharma realizes she has met Rahul's daughter, and that Tina is dead. Rahul soon arrives at Camp Sunshine, after his daughter fakes a fever, where he and Anjali Sharma are surprised and delighted to see each other again, sharing a sweet reunion. Throughout the days spent at the camp, Rahul and Anjali feel themselves falling in love. Just as they are about to share a heated moment, Anjali thinks about her upcoming engagement and feels guilty. She runs away in tears, after which Aman arrives and tells Rahul that he is Anjali's fiancé. Rahul is heartbroken but congratulates Anjali. She takes this as him rejecting her again, so she leaves the summer camp and decides to get married to Aman as soon as possible.

After seeing Tina in a dream, Rahul and his family go to Anjali's wedding, where Rahul silently confesses to Anjali that he loves her, and watches in tears as she is brought forth for the wedding. Realising this, Anjali is hesitant to go on with the marriage, and stands weeping. Aman notices the tears in her eyes and realises that she has always been in love with Rahul. He releases Anjali from the engagement and tells her to marry Rahul. Rahul and Anjali have an emotional hug and as the two marry, little Anjali Khanna sees a vision of Tina, who is smiling and giving her the thumbs up. Rahul and Anjali Sharma get married.

• Shah Rukh Khan as Rahul Khanna: A popular student at St. Xavier's college and Anjali Sharma's best friend. He falls in love with Tina Malhotra and marries her, after which they have a baby girl who is named Anjali. He hasn't realised his love for Anjali Sharma, until he meets her again.
• Kajol as Anjali Sharma: The fun loving tomboy in college and the best friend of Rahul. Over time, she becomes friends with Tina but also falls in love with Rahul. Once she realises that Tina and Rahul love each other, she is devastated and decides to leave the college.
• Rani Mukerji as Tina Malhotra: The principal's daughter, who is a transfer student from Oxford University. She is elegant and sophisticated and described as the most beautiful girl in college. She falls in love with Rahul and marries him, having a daughter with him, but has always felt guilty for coming between Anjali and Rahul. She passes away after giving birth, and her last wish is for Rahul to name their child Anjali.
• Sana Saeed as Anjali Khanna: Named after Anjali Sharma by Tina, she is Rahul and Tina's daughter. She is independent and fun loving and her character mirrors that of Anjali Sharma during college days. She plays matchmaker between her father and his old college best friend, Anjali.
• Salman Khan as Aman Mehra, Anjali Sharma's fiancé
• Farida Jalal as Mrs. Khanna: Rahul's widowed mother who is in devotion towards god and irritates Almeida
• Anupam Kher as Principal Malhotra, Tina's father who is slightly infatuated with Ms. Briganza
• Archana Puran Singh as Ms. Briganza: The college English teacher. She constantly flirts with Principal Malhotra.
• Reema Lagoo as Mrs. Sharma: She is Anjali Sharma's mother and has doubts about her daughter's motives for marrying Aman.
• Himani Shivpuri as Rifat Bi: St. Xavier's girl's hostel matron and motherly figure
• Johnny Lever as Col. Almeida, Half-British manager of Camp Sunshine who is often irritated by Mrs. Khanna
• Parzan Dastur as Silent Sardarji: A Sikh boy at Camp Sunshine who usually doesn't speak, but begs Anjali Sharma not to go when she decides to leave the summer camp. He likes Anjali Khanna.
• Neelam Kothari as Neelam in a special appearance: She is the host of The Neelam Show, the television programme which Anjali Khanna loves.
• Nikhil Advani in cameo and extra appearances
• Manish Malhotra in cameo as college student
• Farah Khan in cameo and extra appearances
• Geeta Kapoor in the song "Tujhe Yaad Na Meri Aayee"

Story

After the experience of assisting and acting in Aditya Chopra's directorial debut, the romance film Dilwale Dulhania Le Jayenge, Karan Johar was encouraged to try his own hand at directing. With Kuch Kuch Hota Hai, he chose to pair up the same lead actors, Shah Rukh Khan and Kajol, for his own romance film. During the filming of Dilwale Dulhania Le Jayenge, Khan had also encouraged Johar to make his own film, and said that he would be willing to star in it.

Johar first wrote a story, which was a love triangle between a tomboy, a very pretty girl and a slightly insensitive boy, but he shelved the idea because he was not very satisfied with it. Then he wrote another plot about a widower and his child, which he shelved as well. Eventually he decided to merge the two stories into one. He explained in an interview with Rediff.com: "It was about the trauma of a widower and his little child. How the child really wants a mother and how she brings her mother into her father's life. Then I thought: Why not bring a youth aspect to the story? Why not a flashback? That's how the story got made." The story also includes a "personal desires vs. parental loyalties" theme, and has some "East meets West" themes, but instead of the characters going abroad, it creates a virtual West inside India.

Development

Karan Johar was certain from the beginning that he wanted to cast Shah Rukh Khan in the lead role, having observed him during the making of Dilwale Dulhania Le Jayenge. It took longer to fill the role of Tina. The role was written with Twinkle Khanna in mind, but she turned it down. Other actresses such as Tabu, Shilpa Shetty, Urmila Matondkar, Aishwarya Rai, Raveena Tandon and Karisma Kapoor were offered the role but also turned it down. Aditya Chopra and Shah Rukh Khan noticed Rani Mukerji's beauty and acting talent in Raja Ki Aayegi Baraat, and suggested her to Karan Johar. He thus signed her, giving a boost to her career. Saif Ali Khan, Ajay Devgan and Chandrachur Singh were initially offered the role of Aman, but they all turned it down, thus prompting Johar to rope in Salman Khan.

Johar, who is also a costume designer, wanted to set a new level for style in Hindi cinema with this film. He and his friend Manish Malhotra, the film's costume designer, made trips to London for costumes, much to the chagrin of his father and producer Yash Johar, who was concerned about the budget. Many of the costumes in the film prominently displayed logos from designers such as DKNY and Polo. In addition to the designer fashions, Johar also created a somewhat fantastical world where the students speak Hinglish and enjoy a pristine college campus, where there is no crime or hate, and traditional Hindu values are pervasive. Johar admitted that the look and feel of the college scenes in the film were patterned after Beverly Hills 90210, saying, "The art, the costumes, tilt toward the West, but the soul of the film is Indian." He hired Sharmishta Roy as art director, and told her to produce something similar to Riverdale High School of the Archie Comics, with some 90210 influence as well. Johar said of the outcome, "If you see KKHH, Shahrukh plays Archie, Rani Mukherji plays Veronica, and Kajol played Betty. It was exactly that. And the principal looked like Weatherbee, and Ms. Grundy was Archana Puran Singh." Johar also hired Farah Khan to do the choreography, Jatin-Lalit to provide the music, Santosh Thundiyil as cinematographer, and Nikhil Advani as his associate director. Shabina Khan assisted Manish Malhotra with costume design.

Filming

Filming began on 21 October 1997. The crew was young and inexperienced to the point where Shah Rukh Khan had to explain basic technicalities of filming. Khan later said, "Karan makes no bones of the fact that his technical knowledge of filmmaking was not at its peak when he made the biggest hit of the decade." The entire film was shot in nine and half months with a substantial part of it shot in Mauritius. The title song was filmed over a ten-day period. in several picturesque locations in Scotland, including Ei