The four part series called Countdown premiered in The Incredible Hulk#364. An unknown party has poisoned Bruce Banner, but before the poison can kill him he transforms into the gray Hulk. At this time, the Hulk's transformation was triggered by the sun setting, so the Hulk has to keep alive while also trying to find out who poisoned him. Because it's a specifically tailored poison, the angrier Hulk gets, the faster the poison spreads, putting him in quite the predicament.
Fortunately the gray Hulk, although significantly weaker, was quite wily and used his brains to outwit such opponents as The Abomination and The Thing. Eventually, he learns that the culprit was a new villain that went under the name Madman, and in his weakened state must fight him to get the cure.