Half Girlfriend movie revolves around the love story of a young couple. Madhav Jha (Arjun Kapoor) has a complex because he cannot speak English fluently. His mother who is  a widow (Seema Biswas)  lives in the village and runs us a school in a small town near Patna. Madhav's dream is to educate children in his village and therefore he comes to town to seek admission in an English college.

 Although he is so weak in English he manages to get admission on the strength of his performance in sports. In college while playing basketball he meets Riya Somani (Shraddha Kapoor) with whom he instantly falls in love. Riya comes from a  very rich family and speaks English very fluently. Their love for sports draws them close to one another. Riya likes Madhav because he is so simple and caring. His friends in college admire him for getting such a rich girl friend and his roommates Shailesh Vikrant  warns him about the difference of class between them 
Riya tells Madhav she would be is half girlfriend more than a friend and less than a girlfriend but Madhav is so desperately in love with her that he  cannot understand the difference. One day Shailesh tells Madhav to decide whether there relationship would go any further by inviting Riya to his room She is very broad minded and sees no harm in spending time alone behind closed doors but she doesn't like the advances he makes towards her in the room. This ends the relationship between Madhav and Riya who walks out completely of his life. He tries to apologize but Riya cannot forgive him as she has seen her father regularly beating up her mother.
 Riya then drops out of college because her marriage is fixed with the rich NRI boy Rohan (Vikant Massey).Riya invites Madhav to her wedding who is heartbroken. She gets married and settles down in Paris.
 Madhav now lives  in his hometown with his mother gets busy trying to expand the education in the village. Madhav meets Riya after 2 years in Patna where he has come to seek a grant from Bill Gates foundation for his school. Madhav and Riya continue meeting each other and their friendship grows. Riya had divorced her husband so Madhav's mother does not like the relationship, again Riya leaves  him  disappears leaving a note sayong that she has cancer and is going to die.
Madhav's work for his Bill Gates Foundation makes him travel abroad where he lives with his friend Rohan.  His heart was always wanting to visit New York where he thought he would find Riya singing in the bar. He starts drinking and  searching. Finally he finds his Lady love singing in a bar, his half girlfriend.