Dil Hai Tumhaara is an emotional tale of an illegitimate child. Unaware that she (Preity Zinta) is the step-daughter of her mother (Rekha), she realises, however, right in her childhood that her mother loves her sister (Mahima Chaudhry) more than her. The emotional bond between the two sisters is, nevertheless, very strong. As luck would have it, both fall in love with the same handsome boy (Arjun Rampal).
The boy reciprocates the love of the illegitimate daughter, but the sister (Mahima) thinks, he is in love with her (Mahima). The mother thinks that her step-daughter is trying to wean away her daughter`s love from her. She is unaware that the boy does not love her biological daughter.
Seeing her daughter`s chances of marriage getting jeopardised, the mother reveals all - that her husband had fathered the other girl and how, after the death of her husband and his paramour, she had raised the orphan as her own child. She accuses her of trying to ruin her daughter`s life just as her mother had ruined her own life.
Distraught, the illegitimate daughter decides to move out of the path of her sister whom she genuinely loves. She even pleads with the boy to forget her and marry her sister instead. Not just this, when the truth about her identity threatens to break the marriage of her sister, she even confronts the boy`s family members and begs of them to not cancel the impending marriage only because of her illegitimate status.This emotionally moves the mother who realises that her husband`s illegitimate daughter is actually a golden-hearted girl. Finally, the sister also gets to know the truth and unites her golden-hearted step-sister with her boyfriend while herself refusing to become the bride.