Uday (Nana Patekar), a criminal don, takes it upon himself to get his kindhearted sister, Sanjana (Katrina Kaif) married, but he is unsuccessful since no one wants to be associated with a crime family. Dr. Ghunghroo (Paresh Rawal) has also been trying to get his nephew, Rajiv (Akshay Kumar), married but due to his condition—the alliance must be with a purely decent family—he is also unsuccessful.
When Rajiv jumps into a burning building to save Sanjana, he is smitten by her. Uday and Majnu (Anil Kapoor), Uday's brother gangster hatch a plan for an alliance with Dr. Ghunghroo. The plan works and Dr. Ghunghroo confirms the alliance, thinking that Uday is a very decent man. But when he is later told that Uday and Majnu are mobsters, he quickly takes his family and flees to Sun City, South Africa to escape. However, Majnu and Sanjana have come to Sun City as well. Rajiv meets Sanjana again and the two fall in love.
Dr. Ghunghroo reconciles with Uday and Majnu and finally agrees to the alliance. Uday and Majnu invite a powerful don of the underworld, RDX (Feroz Khan) to the engagement. At the party, a girl named Ishika (Malika Sherawat) arrives, claiming to be Rajiv's childhood betrothed. Ishika is actually the sister-in-law of Dr.Ghunghroo whom Dr. Ghunghroo asked to come and try to break off the engagement. Ishika manages to do so, leaving Rajiv and Sanjana heartbroken. Dr. Ghunghroo reveals that he did this for Rajiv's mother, who had married into a crime family and was harassed and tortured, thus telling Dr. Ghunghroo to raise Rajiv away from crime when he was born. Dr. Ghunghroo decides he will agree to the marriage only if Uday and Majnu give up their life of crime. Rajiv and Sanjana do this by reawakening Uday's love for acting and encouraging Majnu to pursue his love for painting. With these things keeping them busy, Uday and Majnu have no time for crime any more.
Rajiv's actions anger RDX's son, Lucky, who attempts to shoot Rajiv. Sanjana gets hold of the gun and fires a shot that hits Lucky, causing him to go unconscious. RDX is informed of his son's death and comes to attend the cremation. However Lucky, who is still alive, escapes, trying to show his father that he's actually alive. RDX sets the pile of wood on fire, believing he is cremating his son's body. However Lucky, who had been hiding under the wood, jumps out upon realising the wood is on fire, and the truth is revealed to RDX. Rajiv, Ghunghroo, his wife, Ishika, Uday, Majnu and Sanjana are captured by RDX and brought to a cabin set next to a cliff. The frightened group is forced to play Passing the Parcel (Hot Potato) with a globe—but the one who ends up with the globe must jump off the cliff. When Rajiv refuses to pass the globe to Sanjana, Lucky angrily yanks it out of his hands, just as the music stops. Now that his son has the globe, RDX figures the only way he can maintain his image is by killing everyone. Before he can, several government brokers sneak up and cut the footings of the cabin, causing the house to start falling over the cliff, with everyone trapped inside. However, the cabin is suspended by only one column. Hilarious chaos ensues as the group tries to balance the cabin together and keep it from falling off the cliff. Rajiv finds a rope and the group uses it to get back onto stable ground. But to everyone's shock, the floor breaks and Lucky is found hanging on the edge of the cabin. While Rajiv is trying to rescue him, Sanjana reveals the truth to everyone that she was the one who shot Lucky, but Rajiv blamed himself so that Sanjana wouldn't get in trouble. After Rajiv rescues Lucky, the cabin he is standing on falls off into the cliff. Sanjana keeps crying for him thinking he is dead. However, Rajiv survives the event and is reunited with Sanjana and his family. Lucky and RDX are grateful to Rajiv for saving their lives and RDX gives up his life of crime, allowing Rajiv and Sanjana to finally get married.
The soundtrack was composed by Anand Raj Anand, Himesh Reshammiya and Sajid-Wajid. The lyrics were penned by Anand Raj Anand, Sameer, Shabbir Ahmed, Ibrahim Ashq and Anjaan Sagiri. According to the Indian trade website Box Office India, with around 15,00,000 units sold, this film's soundtrack album was the year's fifth highest-selling.
At the box office, Welcome opened to a massive response locally, by grossing ₹350 million (US$5.5 million) on its opening week. Despite facing competition from the popular Aamir Khan-starrer, Taare Zameen Par which was also a huge success, Welcome grossed ₹710 million (US$11 million) domestically. It became the second highest-grossing film of 2007, just behind Om Shanti Om. The worldwide gross was around ₹1.16 billion (US$18 million).
A sequel to the film, titled Welcome Back has been released. The sequel features John Abraham, Shruti Haasan, Anil Kapoor, Nana Patekar and Paresh Rawal. Earlier reported to release on 19 December 2014, the project finally released on 4 September 2015.