What is this aeroplane - not a Blackbird? What is the aeroplane in the first image?

I noticed this photo in a youtube clip, and at first glance thought it was a three-engine Blackbird SR-71, but it appears to be a single-seater.  



The text and audio does not reveal what the photo is of.


  youtube.com/watch?v=nagVeYYt13s#t=10m41s


Some googling suggests that is a D-21 drone atop a M-21, and wikipedia has this somewhat-unhelpful photo.
The large aircraft is the Lockheed M-21 (which was developed from the Lockheed A-12, the same aircraft that the SR-71 was developed from), and the smaller aircraft is the D-21 drone. 

The wikipedia article on the D-21 has a lot of information.