Following the events from Indestructible Hulk #20 (which ended when Bruce Banner is shot in the back of his head), Banner is unconsciouss and sedated in an operating room. His skull, half open, is waiting for a miracle: Three bullets allocated in his brain need to be removed without killing him. None but Dr. Aaron Carpenter, the best brain surgeon in the planet, may be able to heal him.
SHIELD agents bring Carpenter to the operating room. The Doctor knew Banner from college. He wonders if he could have done something back in the day to prevent Bruce from ever turning into the hurricane of destruction the Hulk has been.
When Carpenter is almost done with the procedure, the agents ask him to place a device in Banner's brain which will allow them to trigger the transformations. "They would use the Hulk as a weapon", Carpenter deduced, and learning the men in black suits were really fake SHIELD agents, Carpenter decides to kill Banner in the table instead. But Banner wakes up in the nick of time, grabbing the doctor's hand. Carpenter realizes the Anesthesiologist stopped delivering anesthesia to save Banner.
When Banner turns into the Hulk, the agents fill the room with toxic gas. It has no effect on the Hulk, but is killing the medical staff. Hulk makes a big hole in the wall to free them, and leaps away. But his brain hurts beyond any tolerance, even his, and collapses in a nearby city.
Two weeks later, SHIELD agents Maria Hill and Coulson arrive at a clinic that has a patient who meets Bruce Banner's description. Fortunately it is in fact Banner, and he's alive, but he has suffered extreme (and "irreversable") brain damage, causing him to lose his brilliant intelligence.