This episode finally shed some light on the past that Charlie and Don had with their mother. And actually told us a little something more about their mother: She enjoyed composing music, and that was the reason that she had the boys take piano lessons as kids.
In addition to that it was a strong episode that tied all of the mathematical loose ends together nicely into a unified whole.
not only did we get a good explaination of what Dr. Fleinhardt does for a living, but the entire episode was sort of an analogy to it.
Here we have a supposedly young, supposedly brilliant scientist, when in actuality he is just trying to steal "one last big score" to retire on. Using the same mathematical models that Dr. Fleinhardt uses to predict his black holes, they discover the not so young man's past and in the end catch up with him
The episode ends on a nice note: Don playing one of the pieces of music that they discovered in the boxes in the garage.