It was originally believed that Reed Richards (Mr. Fantastic) transferred all his knowledge in to Big Brain when he went in to hiding after the death of his wife, Susan Storm (Invisible Woman). However, it eventually becomes apparent that Reed Richards is controlling the Big Brain robot remotely. When Big Brain goes offline, the remaining members of the Fantastic Five, as well as Stinger travel to the source of the remote controlling to find out what happened. They find the origin to be in the Stinger and are attacked when they arrive. They eventually find Reed and he reveals that Susan is indeed still alive, and being held in suspended animation so that she can hold together holes in reality while staying alive. Reed refuses to return home as he wants to find a way of bringing Susan back to life first.
The first version of Big Brain was little more than two pods; one for a body and one with an image of a human brain. Version 1.0 could fly independently and generate force fields and energy beams.
Version 2.0 was a small humanoid but required a hovercart in order to fly and did not emit energy beams.
Reed and Susan recently returned to active duty and the Big Brain robot is no longer a member of the Fantastic Five.