Ae Dil Hai Mushkil is a story of love and friendship that follows Ayan Sengar (Ranbir Kapoor) who is studying Business Management and is also a singer. He lives in London and is the son of a wealthy father. Alizeh (Anushka Sharma), a very rich girl, also lives in London and is a happy-go-lucky girl, very modern in her thinking. Both, Ayan and Alizeh, are in relationships. Ayan has a girlfriend, Lisa (Lisa Haydon), while Alizeh has a boyfriend, Dr. Faisal (Imran Abbas). Ayan and Alizeh meet at a bar one day, quite accidentally, and hit it off. Even as Ayan finds himself attracted towards Alizeh, he and Lisa have a breakup when he catches Lisa in a compromising position with Dr. Faisal. Obviously, Alizeh, who is also there, walks out on Dr. Faisal.
Soon, Ayan is smitten by Alizeh and realises that he loves her. But Alizeh's ex-boyfriend, Ali (Fawad Khan), enters her life once again. Alizeh meets Ali, who is a disc jockey, at a discotheque. Much to Ayan's surprise and shock, Alizeh is willing to give Ali a second chance despite the fact that he had ditched her. Ayan tries to dissuade Alizeh for his own selfish reasons but Alizeh finally decides to marry Ali against her own family's wishes.
Unable to handle the fact that his beloved would henceforth be somebody else's wife; Ayan leaves the wedding midway. At the airport while waiting for his flight, Ayan strikes up a friendship with Saba (Aishwarya Rai Bachchan). Following their meeting, Ayan and Saba spend time together in Vienna where Saba starts falling in love with him. Here, Ayan and Alizeh connect once again with each other. While Ayan has still not been able to get over his love for Alizeh, she still misses his friendship a great deal. Alizeh pays Ayan a visit at Saba's house in Vienna. That is where Saba realises that Ayan would never love her the way he loved Alizeh. Obviously, Saba then has a breakup with Ayan.
Does Ayan re-enter Alizeh's life? Do Alizeh and Ali live happily ever after? Or does Alizeh marry Ayan? Is what forms the rest of the film.