I thought about it for about 10 seconds...
I would bring back the Zygons!!
PLOT:One zygon is transporting a Scarasen seperately to the main ship because they fear about carrying two in one ship, any way, something goes wrong and they end up crashing on Earth... (I know, an Earth story *rolls eyes*) Both Zygon and scarasen are injured and they crawl to safety. There is just enough working technology in the crashed ship for the Zygon to replicate one local. He convinces a local scientific centre to take in the Scarasen but it's condition doesn't change quickly and the Zygon becomes ill.
Enter the Doctor!
The ill Zygon in human form is wierding out the locals, he has very strange symptoms that seem to be spreading. This seems to be a simple case of returning the zygon to its ship and fixing it and helping the scarasen and zygon to heal.
Enter other Zygons!
Oh no! The Doctor seems to be the enemy instead of the helping force he is!
*evil chuckle*
Traditional -> companion gets made in to a Zygon etc Doc gets angry... blah blah blah
Humans get angry and start to attack the zygon scarasen because they think it is spreading disease which causes the Zygons to get angry and take more humans till the Doc is with only a small band and trying to keep peace.
He contacts the Zygon, quotes some shadow proclamation blurb, tells them he can help fix the ships, if they give back the humans etc etc and it results in humans going behind the Docs back, pulling out the humans inside and the zygons shooting at the humans and
BOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOM Simple, traditional and if I knew which companions, easily adapted to the modern day, with no military and humans not working with the Doc and leaving him isolated because his companion(s) have just been put to sleep as they have no use now the Doc knows they are Zygons.
SIMPLE, you did ask and it is late
Ill think you up a better one 2moz