If Raj (Raj Kapoor) had his way, he would have been a poet, not the engineer that circumstances made him into. The perfection of nature fascinated him more than the nature of mechanical things. One day Raj is sent to work at the Saraswati Dam, the exquisitely beautiful location. As Raj revelled in his new found paradise, his father visits him and tells him about his deceased mother's wish that Raj should marry Chandra (Vijaylaxmi), the sophisticated daughter of a rich family friend.

On his father's insistence, Raj agrees to write to Chandra. The letter is sheer poetry, but it's beauty completely wasted on Chandra who wants to ignore it. Neelu (Nargis), her teenage sister, moved by the writer's eloquence and sincerity, acknowledges the letter on her behalf. Neelu's reply sparks off a correspondence and then circumstances that lead Neelu and Raj falling deeply in love. Just when it seems that they will live happily ever after, Raj is diagnosed suffering from TB, a disease that killed his mother.

A shocked Raj decides that he loves Neelu too much to commit her to an uncertain future and deceives her into believing that he actually loves Chandra. Does Neelu ever find out Raj's selfless love for her? Will the two lovers be united again? Aah is an early Raj Kapoor classic, a poignant love story which includes one of Mukesh's finest renditions "Chhoti si yeh zindagani" picturised on Mukesh himself as a "tongawallah", carting an ailing Raj Kapoor.