... is a nice straightforward story with some interesting guest characters and some fun for Rory and Amy, and a cute cliffhanger.
That’s about all one can say, really. There has to be a normal story in the series somewhere.
So I suppose I can talk about what it does in game terms...
“I thought I was going to die!”
“Welcome to my world...”
It's a nice spotlight episode for Rory, where he gets to be heroic - “has to help a damsel in distress” as noted in Confidential - and gets separated and works things out while the Doctor is busy being righteous and Amy is busy being slightly jealous of said damsel. So an interested NPC while the group are split up is a way to shift attention to one of the quieter PCs.
“I am real!”
The Gangers don’t all initially know - if a PC was copied so perfectly, you could borrow a trick and have the copy switch with the normal PC without the player even knowing. If you think the player will be cool with that, and generally just temporarily. If a fake replaces the PC for a longer run, plan this with the player in advance... and maybe keep it secret from all the others, if you think they’ll be cool with it.
“Them or us.”
The Gangers are monsters of the week but they’re not monsters - the TARDIS crew have dropped into something where, so far, humanity is in the wrong.