An unnamed busker in his thirties, credited simply as "Guy", played by Glen Hansard, serenades the streets of Grafton Street in Dublin with his guitar. Amidst the hustle of daily life, he faces the challenges of street performing, including a confrontation with a heroin addict (Darren Healy) who tries to steal his earnings. One day, a young Czech immigrant flower seller, referred to as "Girl" and played by Markéta Irglová, is drawn to his music. Despite his initial annoyance, she engages him in conversation about his songs and insists he repair her broken vacuum cleaner.
The following day, she brings her Hoover to him, turning it into an opportunity for lunch together. As they converse, she shares that she is a musician too. Intrigued, he invites her to a music store where she plays piano. There, he teaches her one of his songs, "Falling Slowly," which she quickly masters, sparking a budding musical and romantic connection between them. He then opens up about his past heartbreak involving a former girlfriend who left him, sharing the story through another song, "Broken-Hearted Hoover Fixer Sucker Guy."
At his father’s shop, she meets his father (Bill Hodnett), who remains indifferent to his son's musical aspirations. When the Guy offers for the Girl to stay the night, her offense causes her to leave. However, they soon reconcile, and during the week that follows, they compose, rehearse, and record music together, significantly strengthening their bond. Their musical journey includes moments where they perform their pieces in the real world, like when the Girl sings her rendition of one of his songs, which she names "If You Want Me."
While their connection deepens, the Guy reflects on his ex-girlfriend (Marcella Plunkett), who has moved to London. The Girl encourages him to pursue a fresh start there. After being invited to her home for dinner, he learns about her life with a toddler (Kate Haugh) and her mother (Danuse Ktrestova). Determined to create a professional demo before relocating, the Guy asks the Girl to join him in this endeavor. They receive a loan from a musically inclined bank officer (Sean Miller) and book a recording studio.
During a romantic motorbike ride on his father's classic 1966 Triumph Thunderbird, the Girl reveals a shocking secret: she is still married, although her husband resides in the Czech Republic. When he inquires if she loves her husband, she responds in Czech, saying "Miluju tebe," which translates to "It is you I love," though she does not share this with him directly.
As they gather a band of musicians (Gerard Hendrick, Alaistair Foley, Hugh Walsh) and enter the studio, their inexperience initially shows, but they soon impress the cynical studio engineer Eamon (Geoff Minogue) with their passion. One late-night break leads to an intimate moment where the Girl shares her own song, "The Hill," expressing her romantic struggles. Despite the realities of her responsibilities with her mother and child, the Guy's feelings for her grow.
After a successful recording session, they walk home together. Before parting, the Girl informs him that her husband will be moving to Dublin. Torn between their feelings, the Guy pleads for her company on his final night in the city; she hesitantly agrees, fearing their meeting would lead to complications. Ultimately, she does not show, leaving him to depart without a proper goodbye. He plays the demo for his father, who is moved by it and offers financial support for his new life in London. Before heading to the airport, the Guy purchases a piano for the Girl and arranges for its delivery, while she awaits her husband's arrival for their reunion.
