The film starts with a wedding invitation in the form of basket full of sweets and chocolates being delivered in numerous places. The invitation to Bunny, Kabir Thapar, couldn't be delivered as he doesn't reside in that place anymore. Naina (Deepika Padukone) receives the invitation and remembers her trip to Kullu and Manali eight years ago. Then she was a shy and nerdy medical student. She studied constantly and always topped her classes, however she felt like an outcast. An encounter with an old classmate, Aditi (Kalki Koechlin), in a supermarket makes her realize that she wants more from life than high marks. She makes an impulsive decision to go on a commercial hiking trip into the Himalayas, up to Manali, even though her seat in the train was not confirmed. On a chance meeting with Bunny(Ranbir Kapoor), he agrees to share his seat with her and take her to a trip with him and his friends. She meets Aditi again and Avi(Aditya Roy Kapur) both of her best friends. Naina, still in the train, still continues to study. She continued to feel like an outcast, as was evident when the game "I never" was being played and she didn't do anything of what was being said by other players (Bunny, Avi, Naina, Lara and her 2 friends).
In the first day of the trip, Bunny, Avi and Aditi leave to explore places in Manali, with Naina tagging along. Bunny continues to tease her with sort of vulgur statements, while Avi continues to drink and flirt. At the end of the day, a gig happens between a lady and Aditi due to Avi's shove mistaken as eve teasing. This ensues a chase, in which Bunny and Naina and running away one side and Avi and Aditi other so that they don't get beaten up from the mob. In the end Naina saves them by making logs fall on the mob when she knocks them down.
At the beginning of the trek, the members are divided into 2 groups, one under Avi and other under Bunny, with Naina and Aditi both in Avi's team. While Bunny is going on slowly to the mountain top, Naina is not stopping in her way. Bunny realizes this may make him lose, tricks Naina into staying with him for some minutes and ties her legs while he busies her in some talk. In the end Bunny's team wins and Avi's team has to prepare food and put tents etc.. Naina had prepared food, but Bunny came and announced that a party of foreigners is going on nearby and they have invited them too. Naina, again feels outcast stays back and resumes her studies till Bunny finds her. He asks her why she didn't join them, she tells him she is friendless and reserved because she is boring. He dismisses the fact that she's boring because she came all by herself to this trip, she sings Bachchan songs and she was the reason they outran those guys chasing them. According to him, she was a hero not boring. He tells her to enjoy more and stop empathizing herself.
As the trip goes on, Naina starts enjoying more and she and Bunny start bonding, with him including her in his gang. The crew reaches the topmost point of their trek, but Bunny wanted to go beyond it. On question, Suber, the trek guide told him that point is called Bhoota Parvat(Haunted Mountain). A myth states that a lots of spirits come there
in the night of Poornima to search for a temple where all their wishes will be answers. Many people tried to find it, but it couldn't happen and they never returned. Coincidentally, that night is also Poornima only.
Regardless of that, Bunny goes there at night with Naina joining him. Naina for the first time drinks with him. Bunny tells her of his dream to roam around the world. He never wants to settle down or have anything to hold back from living his life the way he wants it. He feels marriage and relationship only causes a person to carry a burden of uneventful boring life while he wants his life to be as interesting as possible.
Finally they reach the last point together. Naina realizes she really loves Bunny and wants him in her life. On returning, she feels she has become a new person, a person who thinks more than higher marks and a successful career, a person who can enjoy till the limit.
At the same time, seeing Aditi's reaction to Avi's flirting with other girls, Naina and Bunny realise, she loves Avi.
On the last day, Naina is just on verge of telling Bunny her feelings, Avi barges in and questions Bunny of his admission in Northwestern University in Chicago. Avi found that envelope in Bunny's bag while searching for protection. He comes to know his plans of staying with Bunny in a flat and chilling out were baseless as Bunny was planning to go since a long time. Avi wanted to stop him, but Aditi still angry and hurt by Avi doesn't support him and encourage him.
