Hector (Simon Pegg) is flying one of those old World War II biplanes. His dog is in the backseat and they are happy. Hector does a loop and the dog falls out. Hector realizes and starts screaming. A comic book type villain pops up from the rear of the plane and they begin to fight. Hector wakes up.
Hector and his girlfriend Clara (Rosamund Pike) live a neat and tidy life of habit and order. He is a successful psychiatrist. His rates haven't changed for years and neither has Hector. She works for a drug company and has launched a new pill called Tranqualine.
One nigh, Hector and Clara go to a restaurant to celebrate the success of Tranqualine. Clara's boss is happy that Clara never left them for a maternity leave and is totally devoted to the company without the interference of children.
The owner of the restaurant notices that Hector isn't as boorish as the others. He tells her that he doesn't work for the company. He's a psychiatrist. She tells him she wants happiness, but because of her accent it sounds like wants a penis. Hector and Clara have a laugh about a penis, but truth be told, the conversation makes Hector start questioning his own happiness.
His ordered life now feels like a drag. He begins to lose patience for his patients and eventually snaps. Eventually Hector snaps.
He tells Clara that he needs to go on a journey to research happiness. While Clara packs for Hector she finds an old photo in his sock drawer of a younger Hector with another man and a woman. She doesn't say anything about it. Hector asks if shell still be there when he returns. She asks how long hell be gone. He doesn't know. Neither does she.
Hector flies business class to China. He sits next to a cranky business man named Edward (Stellan Skarsgard). He finds that Clara has packed him a notebook to log his journey and to doodle. She also included the photograph she found in his sock drawer. Edward complains that things are better in first class. Hector notes: 1. Making comparisons can spoil your happiness.
Hector wants to order from the duty free store on the plane, but needs to fill out paperwork. He cant find a pen. Edward lets him borrow his, but tells him that it costs more than his car and he had better return it.
At the airport, Hector fumbles with his map and realizes he still has Edwards pen. He runs to return it and Edward decides to show Hector what his happiness is. He takes him to a nice hotel and a nicer meal. Hector asks Edward what his happiness is. Edward says he works too much to think about it. Happiness is supposed to be retirement and enjoying life. But the only way to enjoy retirement is to not retire. But because Edward works so hard and makes so much money he is able to enjoy the most lavish or luxuries. Hector notes: 2. A lot of people think happiness is being rich or important.
Edward takes Hector to a very exclusive nightclub in Shanghai full of women. Hector meets Ying Li (Ming Zhao) and instantly falls for her hard. He notes: 3. Many people see happiness only in their future.
He also notes after having spent a non-sexual night in bed with a naked Ying Li: 4. Happiness could be the freedom to love more than one woman at the same time.
He takes her out to lunch. There are a group of poor women sitting on the street eating together and having a great time. They're surrounded by a sea of grim businessman, not completely unlike Edward. Hector asks what makes them so happy. Hector asks Ying Li to go with him to the mountains and together they can meet her family. She cant go because its shameful how she makes a living. On cue, we meet her pimp. Ying Li isn't a student. Ying Li is a prostitute. The previous night was a gift from Edward. Hector notes: 5. Sometimes happiness is not knowing the whole story.
Hector goes up the mountains and visits some monks. He asks one how he can be happy even though hes suffered so much in life. The monk explains and Hector notes: 6. Avoiding unhappiness is not the road to happiness.
Hector helps the monks set up their dish network and hes able to Skype Clara. She asks how Shanghai was. Neither can help but to notice how evasive Hector is at answering that question. The wind blows down the radar dish and the signal is lost. The monks dance beneath many colored flags which dance in the wind. They find happiness where Hector is finding grief. Next, he goes to Africa.
Hector flies on a terrifying plane. One of the African women tries to comfort him by noting how old the plane is. That means its never crashed. If they land safely he should meet her family for Sweet Potato Stew. He notes: 7. Does the person you're with bring you predominately A): Up or B): Down?
Hector meets his old friend Michael (Barry Atsma) from the photograph in the sock drawer. He and his bodyguard Marcel take Hector to his hotel. Its a dive. In the bar he meets a drug lord named Diego Baresco (Jean Reno). Diego is upset and quick tempered. He doesn't believe in happiness because his wife is unhappy in spite of taking medication to make her happy. Hector offers to look at her prescriptions. He finds out there may be some harmful interactions between pills and directs Diego to a colleague who may be able to help. He has to borrow Diego's pen though, because he's given Edward's pen back.
Michael sees Hector talking to Diego and explains that Diego is a criminal and should be avoided. Hector still has his pen. He spends a week helping out at Michael's clinic and notes: 8. Happiness is answering your calling.
At the end of the week he finds out that Marcel is more than Michael's bodyguard. Hes his lover. And they're happy. Hector notes: 9. Happiness is being loved for who you are.
Hector meets the lady from the plane and has Sweet Potato Stew with her family. He notes: 10. Sweet Potato Stew!
On the way back to his hotel his vehicle is carjacked and he is kidnapped and locked in a rat infested cell. He notes: 11. Fear is an impediment to happiness.
The kidnappers are about to kill him. Hector says he's friends with Diego, but cant prove it. A gun is pointed at his head, but Hector asks if he can make one final note in his book about what brings his captors happiness. He pulls out his pen to write, its Diego's gold and engraved pen. He is able to prove that he knows Diego. The kidnappers let him go. He notes: 12. Happiness is feeling completely alive.
The previously stuffy Hector would never dance. The reborn Hector celebrates and dances with the woman from the planes family. He notes: 13. Happiness is knowing how to celebrate.
He Skypes Clara, who is getting dressed in a fancy gown. He explains that he was kidnapped. She's disinterested. She has plans of her own and has to go.
Hector flies First Class to Los Angeles. On the plane, he attends to a woman with a brain tumor. The pressure from the altitude is causing her extreme pain. She just needs to get to her sister to say goodbye. The pilot clears the plane for flying lower. When they land, everyone cheers. Hector tells the paramedics not to take her to the hospital, but to see her sister instead. She tells him he's very good at what he does. He explains he's not actually that kind of doctor. She clarifies that she means he's very good at listening. He notes. 14. Listening is loving.
Hector goes to the beach in Santa Monica where the photo was taking with him, Michael and Agnes (Toni Collette). She's pregnant and has two other kids. Agnes, Hectors one who got away, is happily married. He calls Clara to tell her he's in Los Angeles. She says she knew this whole trip was about rekindling an old flame. He explains that she's married and has kids. Hector tells Clara that smothering is mothering with an S. They break up.
Agnes and Hector are going to meet with Professor Coreman (Christopher Plummer) who has written a book on and is studying the effects of happiness on the brain. On the way there, Agnes tells Hector that its unfair and insulting that he put her on a pedestal because she's better than his fantasy because she's real. She did love Hector at one time, but now she loves her husband and her family. He notes: 15. Nostalgia is not what it used to be.
During Coreman's lecture, he points out that people shouldn't be concerned with the pursuit of happiness, but with the happiness of pursuit. Afterwards, Agnes and Hector go to Coreman's lab and check out a project he's working on that will visually show, in real time, the brain and how it reacts to different emotions.
Agnes is instructed to go into a room and think about three things, times when she was happy, sad and scared. She does and through his brain scanning technology, Coreman was able to tell which order she thought about the three emotions. Hector is next.
He thinks about Clara marrying someone else. He thinks about his time being kidnapped and about Ying Li. His emotions are very blocked. Its hard to get a reading. Clara calls and Hector asks that Coreman give him a moment. His brain is still being monitored through the phone conversation.
Clara tells Hector she wants to be a mother, not just his mother. Hector says he's sorry for not explaining his trip and tells her that he learned a lot and the most unhappy thing he could imagine would be to lose her and the happiest thing possible would be to become the man she would want to spend the rest of her life with.
Hectors brain is no longer blocked, all the colors start firing because that's what happiness is. It's everything. We flash to the colored flags the monks were dancing around. They're just like the colors flashing in Hector's head.