Across the Universe

The film begins in an unspecified year of the 1960's with Jude, a young shipyard worker from Liverpool, reminiscing about a girl he once knew and loved on the Liverpudlian beach ("Girl"). A few months prior, Jude enlists in the Merchant Navy and jumps ship in New Jersey, hoping to find his American G.I. father, whom he has never met ("Hold Me Tight", "All My Loving"). Meanwhile, Lucy Carrigan worries about her boyfriend Daniel who is headed for service in the Vietnam War; while in Dayton, Ohio, Prudence pines for a fellow female cheerleader ("I Want to Hold Your Hand"). Jude meets his father, who is a janitor at Princeton University, and befriends Lucy's brother, the privileged and rebellious student Max ("With a Little Help from My Friends"). Lucy receives a letter from Daniel ("It Won't Be Long"), but when Max brings Jude home with him for Thanksgiving Jude becomes attracted to Lucy ("I've Just Seen a Face").
Max drops out of school and he and Jude move into a bohemian enclave in Greenwich Village, living with aspiring singer Sadie (Dana Fuchs). Meanwhile, after his younger brother is killed in the 1967 Detroit riot ("Let It Be"), Jojo is seen leaving home and arriving by bus in New York City ("Come Together"). While Jojo auditions for Sadie's band, Max becomes a taxi driver and Jude finds work as a freelance artist. They are soon joined by Prudence, who has hitchhiked to New York and left an abusive boyfriend.
When Daniel is killed in Vietnam, Lucy decides to visit Max in New York before starting college ("Why Don't We Do It in the Road?"). She and Jude fall in love ("If I Fell"), while Max is drafted into the army ("I Want You (She's So Heavy)"). Prudence is attracted to Sadie, and becomes depressed when Sadie and Jojo begin a relationship. Prudence locks herself in a closet and has to be coaxed out of the closet (literally and figuratively) by her friends ("Dear Prudence"), then disappears after wandering off during a peace rally at which Paco, the leader of the Students for a Democratic Republic (SDR), is a speaker.
At a book function for existential drug guru Doctor Robert, Jude, Lucy, Jojo, Sadie and Max drink punch laced with LSD. They embark with Doctor Robert on his "Beyond" bus ("I Am the Walrus"), end up stranded outside the compound of psychonaut, and see Mr. Kite's bizarre circus where they are reunited with Prudence, who performs as Henry the Horse ("Being for the Benefit of Mr. Kite!", "Because").
Back in New York, Sadie reluctantly agrees to her manager's demand that she drop her backing band, leading to a bitter breakup and musical split between herself and Jojo ("Oh! Darling"). Max is deployed to Vietnam, leading Lucy to become increasingly involved in the anti-war movement, especially with the SDR. Jude remains comparatively apolitical but devoted to Lucy ("Something"). Jude dislikes the increasing amount of time that Lucy spends with the SDR and suspects that Paco is attempting to seduce Lucy, and this puts a strain on their relationship and affects Jude's art ("Strawberry Fields Forever"). Finally, Jude storms into the SDR office and points out the hypocrisy of the group's actions ("Revolution"), leading to an argument with Lucy in which she leaves him ("While My Guitar Gently Weeps"). Jude follows Lucy to an anti-war demonstration at Columbia University ("Across the Universe") but when the police start arresting the protestors (including Lucy and Paco), Jude tries to help Lucy but is beaten by police and arrested ("Helter Skelter").
Having been in the United States illegally and unable to legally prove that he is the son of an American citizen, Jude is deported back to England, where he returns to his old job at the Liverpool shipyards ("A Day in the Life (instrumental)"). This is also revealed to be the time frame of the opening scene. Jojo continues his music, playing solo guitar in bars, while on tour (although her career is skyrocketing) Sadie drowns her sorrow and loneliness in alcohol. Max is wounded in Vietnam and sent home, psychologically scarred and dependent on morphine ("Happiness Is a Warm Gun"). Lucy continues her activities with the SDR, but finds herself caught up as Paco leads the movement deeper and deeper into violence. She finally leaves when she discovers that Paco is making bombs, but is surrounded by constant reminders of Jude and what they had shared ("Blackbird"). One of Paco's homemade bombs explodes, killing him and his confederates and destroying the SDR offices; on reading this news in the local newspaper, Jude thinks that Lucy is dead, but upon learning from Max that she is alive, he arranges to return to New York legally ("Hey Jude").
Jojo and Sadie, who have reconciled, put on a rooftop concert, with Prudence as a member of their band ("Don't Let Me Down"). Max brings Jude to the rooftop, but the police arrive to break up the concert and Lucy down on the street cannot get through the barricade. Jude manages to remain on the roof and begins to sing ("All You Need Is Love"). The police allow the band to accompany him. Max draws Jude's attention to an opposite rooftop where Lucy is standing and looking at him. Lucy and Jude gaze smilingly at each other across opposite rooftops as the performance concludes ("Lucy in the Sky with Diamonds").
When Netflix released the movie to the public, the scene containing Paul McCartney's song 'Let It Be' was deleted in its entirety. This was, and is, a very pivotal moment in the movie as it portrays the actual violence happening in America during the Vietnam War. 'Let It Be' is an anthem cry for help, a call for answers and wisdom in troubling times.

Question: Why does Prudence become depressed?
Answer: Because Sadie and Jojo begin a relationship