This is an effect in movies and tv where there are two groups of
people and by the end of the movie or show each
person in one of the groups has paired up romantically with someone from the other group. For example how in the musical
Grease each of the T birds by the end of the movie is dating one of the pink ladies.
This movie used the
Grease Effect to pair up all of the characters.
The Grease Effect was used in White
Christmas so that each of the guys could end up with one of the
girls.