you don't have to take "crossing the boundary" literally.
"Crossing the boundary" can be use to illustrate something which is
Taboo, ie. falling in love with your cousin, insubordination,
definance against your parents or superiors, etc.
if you really want to, a good idea is to write about a prisoner,
heard of the news of her old mother's cancer, dying in a few days
time. Main body of the essay is about his plan to escape from
prison, "crossing the boundary" of the prison wall/guards. Write
about his thoughts, emotion, and finally his recapture.