For the Sake of the Plot

A reason given when something unrealistic, unbelievable, or too convenient happens in a movie, book, or TV show because otherwise the plot would be ruined. Especially relevant when characters miss obvious details that would lead to big revelations.

"Why the FUCK hasn't Lois Lane realized that Clark Kent is Superman! They look exactly alike and he DOESN'T EVEN WEAR A MASK!"
"For the Sake of the Plot."