Reflecting on the process of coming of age, the EP departed from the narrative explored in the preceding three releases and focused on self-expression and introspection.

With members Jaehyun, Taesan, and Woonhak credited for writing the EP's tracks, 19.99 explores the anxieties of the youth before they turn twenty.

[6][7] 19.99 opens with the raucous hip-hop track "Dangerous", which narrates a nighttime escape and expresses themes of rebellion and youth.

[6][11] The closing track, "Call Me", was heavily inspired by a "comforting text" that Taesan received from his father, which later became the basis of the song's chorus.

[12][13] The 16-second teaser trailer features a push-button telephone lying on a puddle, rapidly counting down before being picked up by the members who then walk away.

[16][17] Images from themed photoshoots depicting an album concept—"Nice", "Dangerous", and "Twenty"—were released and featured in promotional material for the EP.