The Doctor (Ncuti Gatwa) explains how the TARDIS works to his new companion Ruby Sunday (Millie Gibson). They travel 150 million years back into primeval Wyoming. Ruby steps on a butterfly and changes into something unhuman, but the Doctor saves the butterfly and she changes back. Into human shape.
Then they travel to a space station in the future, where there is a monstrous creature. They take an elevator to a higher level, which is a baby farm. It is run by talking babies who live in terror of the creature below, which they call the Bogeyman (Robert Strange).
The babies are taken care of by an AI named Nan-E, which turns out to be programmed by a woman named Jocelyn (Gold Rosheuvet), the last of the original ship’s crew, who had abandoned ship, leaving the babies behind. The Doctor discovers a refugee planet nearby and intends to send Jocelyn and the babies there. He has a vision of himself involved in Ruby being abandoned by her mother, which he finds disturbing.
A baby named Eric (Mason McCumsky and voice of Sami Amber) faces the Bogeyman, and the Doctor, along with the babies, have to come to his rescue. The Doctor finds himself deathly afraid of the Bogeyman. He learns that the ship created the Bogeyman to amuse the children and it is made of real Bogeys—that is, mucus.
Jocelyn has the Doctor and Ruby lead the Bogeyman to the air-lock, but before they can space it, they learn that it was made from a baby. Ruby prevents its destruction and the babies recondition it to a kind of Doggie.
The Doctor repairs the ship so it can be flown to the refugee planet, gives a TARDIS key to Ruby and invites her to travel with him. But they cannot visit her abandonment because they might be tempted to change events, creating a paradox. The Doctor performs a secret DNA scan of Ruby and leaves the TARDIS, which is snowing inside.
The story was written by Russell T. Davies and directed by Julie Anne Robinson. Some of the scenes shadow scenes in Davies’ first episode, The End of the World, in 2005. The babies were played by real babies and dolls, as it was a bit dangerous to give flamethrowers to babies. The fake snot was made of liquid latex, resin, and glycerine, warmed by a hairdryer and painted. The episode received good reviews, generally, on Rotten Tomatoes, though some called it forgettable.