It's nighttime. The Numberblocks are asleep. Four cannot make up his mind whether he can stay up or go to sleep. Four has trouble deciding.
The two Twos become the Terrible Twos!
The Terrible Twos start by tickling One, causing her to laugh. Then they shout "BOO!". Frightened number One bounces on her bed. Next, the Terrible Twos tickle the normal Two. When they shout "BOO!", Two is frightened he separates into two Ones.
Next, the Terrible Twos try to tickle Three, but she wakes up quickly and juggles just before she gets tickled. When the Terrible Twos shout "BOO!", Three separates into three Ones. They chase the Terrible Twos to Four's bed. They shout "BOO!" to themselves, causing them to separate into four Ones.
One remains as one block.
Nine other Ones turn back into Two, Three, and Four. Five wakes up and wonders what the noise was. Four explains that he was the Terrible Twos. The others think that he's joking and go back to bed.
The Terrible Twos call out their name and shush to the camera.
4 stays in a square shape for the entire episode.
Before the release of "Once Upon a Time", BlueZoo uploaded a GIF of The Terrible Twos and Four on a white background flying in the air from Peekaboo on Twitter.