Marfa Girl follows Adam (Adam Mediano), a directionless 16-year-old boy, in the days surrounding his sixteenth birthday. The film details his relationships and interactions with the other inhabitants of the small Texas town, including his 16-year-old girlfriend Inez (Mercedes Maxwell), his 23-year-old neighbor Donna (Indigo Rael) who attempts to seduce him as a sixteenth birthday present, his teacher (Lindsay Jones) who is eager to deal out corporal punishment, and a newly arrived local artist (Drake Burnette) who also seeks to have sex with Adam. A sub-plot involves Tom (Jeremy St. James), a mentally unstable border patrol agent who becomes fixated on Adam's mother (Mary Farley), as well as on Adam's girlfriend and Adam himself.