You can actually use the Freebase API (in particular, the Topic API) to do something like this. Basically, you want to fetch the /common/topic/description
attribute, like this:
https://www.googleapis.com/freebase/v1/topic/m/02mjmr?filter=/common/topic/description
(You can also use Freebase to get most of the other attributes that display in the Knowledge Graph).