fall in phrasal verb (
see also fall)
1. if the roof, ceiling etc falls in, it falls onto the ground
Synonym : collapse2. to start walking or forming a line of people behind someone else
fall in behind
His men fell in behind him. [TahlilGaran] Dictionary of Contemporary English ▲
fall in v. phr. To suddenly make sense; find the natural or proper place for the missing pieces of a puzzle.
When the detectives realized that a second man was seen at the place of the murder, the pieces of the puzzle began to fall into place. [TahlilGaran] English Idioms Dictionary ▲