Mr. Bean (Rowan Atkinson) is a well meaning, but hopelessly clumsy and destructive security guard, at the National Gallery in London. His reputation for sleeping on the job compels the board of directors to attempt to fire him, but they are thwarted by the chairman (Sir John Mills) who is sympathetic to him. In order to get rid of Bean, the board send him to Los Angeles to represent them at the unveiling of the portrait Whistler's Mother. The famous painting has been purchased for $50 million by the Grierson Art Gallery with a grant from General Newton (Burt Reynolds).
Bean's visit is arranged by the gallery's curator, David Langley (Peter MacNicol), who, impressed with the National Gallery's false profile of "Dr. Bean", agrees to accommodate Bean in his house for two months, much to the chagrin of his wife Alison (Pamela Reed), son Kevin (Andrew Lawrence) and daughter Jennifer (Tricia Vessey), who subsequently leave for Alison's mother's house.
After some initial mishaps with the airport police and leaving the gallery's owner, Mr. Grierson (Harris Yulin) slightly doubtful about Bean's intelligence after meeting him in person, David begins to question his decision, and his worst fears are realised when Bean accidentally ruins the painting shortly after it arrives. Fearing that he will lose his job and possibly face criminal charges for the damage, David becomes despondent and gets drunk, even though his family returns out of pity.
Bean, however, comes up with a plan to save David's career by sneaking into the gallery at night and replacing the damaged painting with a poster that he alters to make it resemble a genuine painting with egg whites and Alison's nail varnish. The plan works and the painting is a success, but Bean is unexpectedly called to give a speech about the painting with General Newton, countless journalists and reporters watching. Bean, however, manages to improvise a sentimental and deep monologue about the painting and wins the crowd's praise and approval.
Just then, however, David is visited by Lieutenant Brutus (whom Bean had crossed more than once earlier on) and is initially perplexed with the police presence, assuming that the police might have discovered the issue with the priceless painting. Brutus however tells David that his daughter Jennifer has been in a motorcycle accident.
They rush to the hospital, but Brutus and the police stop prematurely to deal with a mugging, in which Brutus is shot. David shares an uneasy reunion with Alison beside the unconscious Jennifer's bed, while Bean is mistaken for a medical doctor and forced into a surgery room to help remove the bullet from Brutus's chest, which he manages to do unorthodoxly. While still in the doctor's garments, Bean comes across David, who asks him to help awaken Jennifer.
Once alone with Jennifer, Bean begins fooling around the defibrillator and zaps himself, accidentally causing him to wake Jennifer. Bean uneasily reveals himself to David and Alison and asks that he be allowed to stay with them another week, which they gladly accept. Bean goes on to spend quality time with David and his family until he leaves.
Eventually, David takes Bean to the airport for his flight home, where they part ways as friends. Once at home in London, Bean takes one last look at his room, which he has decorated with photographs with the Langleys and the original Whistler's Mother that was damaged and then stowed away, before going to bed.

Question: Why does David become depressed?
Answer: Mr. Bean ruins the painting. 
Alternative answer: scared of getting fired and aressted