A great bit of worldbuilding, a terrifying concept for a 'monster', a quick environmental message, and a beautiful ending. This is just a really great short story, the kind you could read a kid just before bed for them to doze off to the end of.

The characterisation is solid here too, especially for 10 who tends to get the short end of the stick in Martha stories (presumably given how he treats her for a lot of that TV run which is understandable), but that last moment here with his is just lovely.