The opening credits show us How We Got Here: Once upon a time, General Thaddeus "Thunderbolt" Ross (William Hurt) was trying to recreate a "Super-Soldier serum" which was used by American soldiers in World War II. He turned to the boyfriend of his daughter Betty (Liv Tyler), nuclear physicist and biochemist Dr. Bruce Banner (Edward Norton), to try and reconstruct the program. During the experiments, Banner was exposed to gamma radiation
and became afflicted with a Jekyll & Hyde problem: if his heartbeat ever exceeds 200 bpm, The Smart Guy transforms into The Big Guy: The Hulk, a very large green man overwhelmed with both muscles and Unstoppable Rage. Hulk smashed his way out of the lab, injuring both General Ross and Betty, and became a fugitive from the US, while Ross tried to capture him in an attempt to weaponize the technology.
Five years later, Banner is hiding out in Rio De Janeiro and working in a bottling factory, while collaborating with colleagues over the internet to find a cure for his condition. He has been studying various calming and meditation techniques — yoga has been particularly effective lately — to avoid transformations. However, a workplace injury leads a drop of Banner's blood to fall into a bottle of soda; it is ingested by a civilian in Milwaukee, WI, providing the first proof in five years that Banner is still out there. Thunderbolt Ross authorizes an American special forces team, led by Emil Blonsky (Tim Roth), to apprehend Banner, but Banner Hulks out and escapes.
Bruce returns to America and reunites with Betty, though for safety reasons he must keep his excitement under control. Almost immediately Blonsky and a new supply of mooks try to apprehend him, but Bruce defeats him, and he and Betty go on the run. They reach out to Bruce's colleague, Samuel Sterns (Tim Blake Nelson), who urges Bruce to come meet him: he has made what he believes is a cure for Bruce's condition. Alas, it's a trap; Bruce and Betty are both captured. Additionally, Blonsky, smarting over how often Banner has managed to escape him, persuades Sterns to inject him with some of Banner's blood, causing him to become "The Abomination." His flailing around causes Sterns to also get some Hulk blood on a cut to his forehead and begin to transform in some way, a Chekhov's Gun that was Left Hanging for over a decade.
The Abomination goes on a rampage through Harlem, completely beyond the abilities of conventional arms to subdue. So Bruce convinces General Ross to let him go down and put a stop to things. After a CGI-heavy fight, Hulk wins, having The Abomination at his mercy and choking him with a chain; he also shows that The Hulk is not beyond total control, as Betty pleads for him to spare Blonsky's life and Hulk is able to listen, leaving The Abomination subdued and to US custody. He then leaves NYC.
A month later, Banner has begun to take conscious control of his transformations. Meanwhile, Ross is Drowning His Sorrows in a bar when he's approached by Tony Stark (Robert Downey Jr.) to talk about a Super Team being put together…
