Preeti is a young medico living in Mumbai along with her widower father, Heerachand. On a holiday to Simla, she meets a young man Prem, an amateur skater. He teases her initially but later falls for her. After some wooing, Preeti also reciprocates his feelings. During a trip to the hills, Preeti meets with an accident and loses consciousness. To save her from hypothermia, he gets close with her and one thing leads to another and become intimate.
Immediately after, Prem tells her that he got close only to save her and proposes to her. She gladly accepts and directs him to her father. Her father, Heerachand doesn't want her daughter to marry him and offers him money to leave her. Prem doesn't agree and leaves a letter for Preeti at her hospital explaining everything and asking her to come with him. But Heerachand meddles with the letter and rewrites it as if Prem was asking money to marry her. Preeti becomes surprised and decides to forget him and leaves for Bombay.
There, she gets engaged with Dr. Amar, her long-term friend and admirer. He leaves for Germany for higher studies and then Preeti learns of her pregnancy. Her father wants to abort the child, but she doesn't agree, at last, they decide to go to Khandala to give birth. She goes there and unbeknownst to her, Prem stays there along with his mother. Preeti gives birth to a healthy boy there. Knowing that Prem requests to take the child for himself. Heerachand agrees and gives him the child. Preeti is told that the baby was stillborn. She mourns and returns to Bombay.
Prem comes to Bombay along with his son and starts a career as a skating teacher. His son Rahul can't walk due to a polio attack. After years, Prem again meets Preeti in a function. She thinks that Prem got married and had a son. In that function, Amar announces that he wouldn't marry till he makes Rahul walk. He starts treating Rahul and meanwhile Preeti grows attached to Rahul without knowing that he is her son. She tries to talk to Prem, who then knows the truth that Heerachand changed his letter. But he keeps mum for the sake of Amar.
Amar learns the truth about Preeti and Prem and becomes troubled. He succeeds in making Rahul walk though. Prem gets mortally injured while saving Tikki (Amar's sister) from goons. He gives Rahul his wallet before going for surgery saying that his mother's photo is in there. Preeti sees it and questions her father about her child. He confesses that he hid the truth from her and Rahul was her son. Amar saves Prem and everybody reconciles. Preeti and Prem marry and stay happy with their son.
[ https://en.wikipedia.org/wiki/Aa_Gale_Lag_Jaa_(1973_film) ]