A quasi-documentary portrait of young non-actors striking poses, walking around Boston, hanging out, and playing or listening to music, Damien Chazelle’s giddy avant-musical is at once fresh and retro, a casual three-mumblechord affair glamorized by its exuberant nostalgia for pop bop of the late ’50s and the then-New Wave excitement of Shadows and Breathless. Narrative barely exists, except as musical-comedy trope: Self-absorbed young trumpeter Guy (jazz musician Jason Palmer) and beatific, somewhat bewildered Boston student Madeline (Desiree Garcia) fall in love, break up, become involved with other people, and reconnect . . . maybe. The movie was shot old-style in black-and-white 16mm, nervously hand-held and mainly in tight close-up. Not just Justin Hurwitz’s big-band score but the entire movie jumps. Music and dance are all the more crucial in that the characters are otherwise notably non-communicative. The big production number is staged at the Summer Shack where Madeline works as a waitress. Fellow employees join together to clean the joint and tap on the tables as she recaps her story: “I kissed a boy in the park! . . . I like New York in the fall!” The enthusiasm with which Chazelle and company put on this show is anything but innocent, but it is infectious. Guy and Madeline is at once self-conscious and breezy, clumsy and deft, annoying and ecstatic. It’s amateurish in the best sense. No movie I saw last year has given me more joy.