Bold and thought-provoking, this contemporary romantic drama delves into the relationship between two couples Dev and Maya, and Rhea and Rishi. Only, Dev is married to Rhea and Maya is married to Rishi.
Dev Saran (Shah Rukh Khan) is a successful soccer player in the United States. He lives in New York with his wife Rhea (Preity Zinta), a fashion designer, his young son Arjun, and his mother Kamaljeet (Kirron Kher). Maya (Rani Mukerji) is an orphan who is to marry her closest childhood friend Rishi Talwar (Abhishek Bachchan). The two have been raised by Rishi's fun-loving father Samarjit a.k.a. Sam (Amitabh Bachchan).
Dev encounters Maya moments before she is to marry Rishi, and the two have a deep conversation. Although Dev and Maya are strangers, they connect instantly. When Dev bids Maya goodbye, she tells him they should never say goodbye because they may meet again. Moments after they part, Dev is hit by a car and permanently injures his leg; as a result, he is no longer able to play soccer.
Four years later: Dev is now extremely bitter – first because of his inability to play soccer and second because Rhea is very successful in her career, making Dev feel inferior. Maya is in a similar situation: she has found that she is infertile and does not feel any love towards Rishi because she believes he is selfish. The two couples meet and quickly become friends.
When Dev and Maya witness Sam and Kamaljeet embracing, they confront them about it. Sam and Kamaljeet explain that having both lost their spouses, they understand each other and have decided to help one another through their loneliness by means of friendship. Dev and Maya notice each other's marriages failing miserably and, taking into account what Sam and Kamaljeet said, they decide that they need to help each other save their marriages.
They meet every day for coffee and discuss their situations. Eventually, Dev and Maya decide to take a break from the stress of their relationships and spend some quality time together. In doing so, they find themselves drawn to one another.
After celebrating their wedding anniversaries, both couples have an argument. Upset, Maya arrives at the train station and finds Dev, who finally confesses that he has fallen in love with Maya, and so has she. Rhea and Rishi talk with each other and realise they need to stop arguing with their spouses and reach out to them.
For a while, both marriages seem to be fine; however, Dev and Maya begin an affair, as they are happier with each other. The two couples attend a ballet show, where Dev and Maya try making each other jealous by showing affection to their partners. The next day, they realise what they are doing is wrong but nevertheless give in to their love and sleep together.
Sam invites the Sarans to dinner to celebrate Kamaljeet's birthday. Here, Dev 'jokingly' admits he loves Maya; Sam is the only one who realises that Dev was being serious. Shortly afterward, Dev and Maya are caught together by Sam and Kamaljeet. Sam has a heart attack; on his deathbed, he advises Maya to leave Rishi, as neither of them is happy in the relationship and never will be.
After Sam's death, Dev and Maya decide they must end their relationship and go back to their respective spouses, but must tell them the truth first about their extramarital affair. Shocked and heart-broken, Rhea and Rishi decide they have had enough and divorce their spouses. Dev and Maya lie to each other and say that they have managed to sort things out with their spouses, believing they'll never meet again.
Three years later: Dev and Maya now live alone miserably, and have not seen each other. Rishi visits Maya and says that he has fallen in love again and is remarrying, and wants Maya to be his best man, to which she agrees. Rhea, having also been invited, arrives at Rishi's wedding with her boss Jay (Arjun Rampal in a cameo), whom she is now dating.
Rhea learns that Rishi and Maya are divorced – Rishi learns the same about Dev and Rhea. Maya is encouraged by Rishi and Rhea to go to Dev, who is leaving for Toronto by train, as they have both moved on and believe Maya should find her true love.
At the station, Dev sees Maya but tries to avoid her, believing she is still with Rishi. However, Maya finds him just as the train leaves. Dev pulls the emergency stop and arrives back at the station. He learns that Maya has also been divorced and alone in these three years. They finally find happiness with their love and decide to start a new life together. At the end, Dev narrates that love should be the only foundation of marriage but it shouldn't be at the expense of hurting others.