The Mystery of the Missing Six

Plot Summary
Right after a mishap happens, Five snaps at Six and Six wishes that she could just run away from home in the meantime. One tells Five that he just upsetted Six's emotions and their mother folds her arms and isn't amused about it at all. When Six seemingly goes missing, the 6 monsters think that Six has gone off to the hospital to give herself back when it makes Five cry a bit. The search goes on when Five learns that SIx is always needed here instead of being sent away, but  When he begins crying a bit, he says to them that she wished not to be here any longer and One, Two, Three, Four and Seven begin crying a bit so it's up to them to get Six back here where she belongs.

Trivia Notice
Three returns again as Sherlock Three in this episode.

Transcript
The Mystery of the Missing Six transcript