A boy named Rolfe comes to the door with a telegram for the captain.
When Liesl, the captain's eldest child, learns it was Rolfe who delivered the telegram, she sneaks away to the gazebo, where Rolfe's waiting. They embrace. Apparently they're… friends.
In between their romantic chat, we learn that Austria's in transition—and Rolfe believes that Captain von Trapp is not on the "right" side of things. That is, he's not into the idea of Austrians becoming Germans.
Since Liesl is only "Sixteen Going On Seventeen," Rolfe sings her some advice about the grownup world. After all, he's seventeen going on eighteen.
Liesl seems to welcome the advice; she knows she's naïve. She needs a man to take care of her.
They move into the gazebo when it starts to rain.
At the end of the song, Rolfe kisses her. He leaves and she squeals with happiness.