In south inner-city Dublin in 1985, the Lalor family has some problems. Patriarch Robert (Aiden Gillen) is struggling with his architecture practice and his marriage, and drinks and smokes to excess. At a family meeting, he announces that in order to save money he is taking the film’s protagonist – his youngest son Conor (Ferdia Walsh-Peelo) – out of his expensive fee-paying school and moving him to a free state-school, Synge Street CBS, which Robert asserts is of equally high repute. Conor’s elder brother Brendan (Jack Reynor) ribs him about the change as well as about the state of the family unit.
Conor appears in school on the first day in his new uniform, but without the regulation black shoes. The main antagonist, school principal Br. Baxter (Don Wycherley) takes him to task for this despite Conor’s pleas of being unable to afford new black shoes, eventually forcing him to complete the day shoeless. The shoe problem is resolved by painting the shoes black using paint from the art room. An encounter with the school bully introduces Conor to Darren (Ben Carolan) an ally and budding entrepreneur. When Conor tells the beautiful Raphina (Lucy Boynton), in order to impress her, that he needs a model for a music video his band is making, Darren agrees to manage the outfit and introduces him to multi-instrumentalist Eamon (Mark McKenna).
The band practice in Eamon’s living room, playing a bunch of 1980s covers. But Conor has a sibling-induced epiphany when Brendan encourages him to drop the covers and develop the band’s own style. He begins writing original songs with Eamon. The band film their first music video for their song “The Riddle of the Model”, wearing a motley assortment of often comical costumes, with Raphina acting as ingenue and makeup artist. Raphina gives Conor the name “Cosmo” which she says is more in keeping with his new band’s image and the band continues to write and record new songs. After spending the day filming a music video for a new song, Conor and Raphina kiss and exchange confidences.
At home Robert and Penny’s (Maria Doyle Kennedy) marriage is falling apart, while Raphina and Cosmo’s relationship blossoms, leading to Cosmo taking her out to Dalkey Island in his granddad’s motor cruiser. Here they view the car ferry leaving Dún Laoghaire for Britain and discuss Raphina’s dream of leaving Ireland for London.
An opportunity arises for the band to play live at an end of year party at school. Cosmo prepares a new song mocking Baxter, which the band perform as an encore while distributing homemade masks of Baxter’s face, leaving the party and school in uproar. Cosmo and Raphina persuade Brendan to drive them to Dalkey so they can escape in the motor cruiser and head out to sea following the ferry across the rough Irish Sea to a new life in London.