When the Doctor leaves, Alex catches up to him. He has some lingering concerns about raising an alien. "He's a Tenza," the Doctor reminds him. "He'll adapt perfectly, be whatever you want him to be." The Doctor says this with a big smile, even though it's a pretty awful notion. Human children can struggle against their parents' expectations, chafe against their destinies, forge their own identities. Not this boy. It's the saddest, creepiest thing about the episode. Or, rather, it would be if it had been anything more than a throwaway line.
The Doctor and the Ponds head back into the TARDIS. As they discuss where/when to go next, a creepy doll voice begins to sing "Tick tock goes the clock, even for the Doctor." The monitor above the console shows the Doctor's date of death, in case the crushing boredom from this episode made you forget.