Incredible Hulk #4

Rick uses some of Banner's machinery to transform the Hulk, who was stuck in his monster persona, back into Bruce; Bruce is incredibly weakened by the transformation and takes a chance in using the machinery on himself again which transforms him back into the Hulk's body but with Bruce Banner's mind; Now Bruce and the Hulk can switch at will using the gamma ray machine.

An alien invades earth and claims his people will conquer the planet unless he is met in battle by the strongest opponent the world has to offer; Hulk meets the challenge but finds out it is all a ploy by the Russians to capture him and create an army of hulks of their own.