‘Sanam Teri Kasam’ starts off with everybody congratulating the leading lawyer Inder Lal Parihar aka Inder (HarshvardhanRane) because he has won an important case. Steering away from everyone and all the congratulatory messages, Inder stands in front of a huge tree, which leads the viewers to a series of flashback incidents that connects Inder's yesterday with his today. The female protagonist Saraswati (MawraHocane) is a simple librarian whose plain appearance repels suitable men. Her parents attempt to find a groom for her but in vain and insist that her sister Kaveri will only be married after she is wed. Encouraged by her sister, who desperately wishes to wed her boyfriend Sanjay, Saraswati decides to get a makeover.
She seeks help from Inder (HarshvardhanRane), her neighbour, whose girlfriend is a renowned stylist. His girlfriend walks in on them speaking and presumes that Inder is cheating on her, promptly retaliating by throwing a glass bottle at Saraswati that strikes Inder instead. Saraswati aids Inder in seeking medical care and guides him to his apartment. A security guard alerts her father of this development, accusing her of adultery. He disowns her in anger, following which she promises to fulfil their hopes by finding a suitable groom for herself.
Inder helps her get a makeover and she attracts the attention of Abhimanyu, her colleague. Inder falls in love with Saraswati during this time; on the day of her marriage to Abhimanyu, she is left at the altar. Saraswati rushes to Abhimanyu's house, where his father reveals that he does not want a disowned daughter in law. She leaves heartbroken and travels to her honeymoon suite, where she asks Inder to make love to her for consolation, to which he ultimately agrees. The next morning, Inder wakes up to find Saraswati missing and witnesses her embracing Abhimanyu. This leaves Inder heartbroken and he spirals into depression. At a later date, Saraswati's mother asks Inder to give Kaveri's wedding invitation to Saraswati.
He travels to Abhimanyu's office and learns that Saraswati had actually left Abhimanyu and requested him to not inform anybody about it.Inder searches for Saraswati, tracks down her phone and finally finds her in a Buddhist monastery. She tells him to leave and claims to use him to get over Abhimanyu. He leaves heartbroken, but he comes back to give Kaveri's wedding invitation and professes his love for her upon seeing her tears. She faints, and at the hospital, Inder discovers that she has a malignant tumour that is untreatable. Saraswati tells him that she desires to be buried under a tree so that flowers can be showered upon whoever visits her grave. Later that day, Inder forcibly enters the house of Saraswati's parents and demands that they stop pretending their daughter is dead. The next day he is arrested for charges pressed by her father. While he is at the police station, Saraswati gets discharged from the hospital and goes to the library where she discovers the secret messages that Inder left for her in check-out slips. Inder runs from the station to find her, and after witnessing his desperation, the inspector allows him to meet her at the library. They unite in an embrace and Saraswati asks him to marry her. The inspector encourages their unison.
While she is sleeping that night, Inder goes to her parent’s house again and threatens to kill her father, saying that he can kill or die for her happiness, announcing their marriage. During the wedding the following day, Saraswati's parents forgive her and bless the occasion. She faints after signing the marriage document; the hospital informs her family that she will die imminently. Inder asks his father, whom he hates, to acquire for him the land under the tree for Saraswati's burial but leaves when his father misunderstands him. Saraswati dies in his arms, promising to meet him soon. Inder leaves the ward to find his father waiting, and after their respective apologies, they reconcile.
In an epilogue scene, Inder is shown standing near the tree that marks Saraswati's grave. He asserts that she must be happy to see him successful and sober, confessing that he has left drinking. He confesses that he misses her very much and every day of his life. Inder expresses his gratitude and a promise of eternal love as a rendition of title plays in the background amidst showers of flowers.