Ethan Hunt is alerted by the IMF that someone has used his identity to assist bio-chemical expert Dr. Vladimir Nekhorvich to enter the United States, only to kill him in a subsequent plane crash. Nekhorvich, an old friend of Ethan, had forewarned the IMF of his arrival, planning to deliver to them a new bioweapon, Chimera, and its cure, Bellerophon. He was forced to develop these by Biocyte Pharmaceuticals. IMF determines that rogue IMF agent Sean Ambrose is responsible. IMF assigns Ethan to recover the virus and its cure. It also insists that he recruits Nyah Nordoff-Hall, a professional thief presently operating in Seville, Spain. Later, Ethan finds out that she is Ambrose's ex-girlfriend.
After recruiting Nyah, Ethan assembles his team, computer expert Luther Stickell and pilot Billy Baird, in Sydney, Australia, where Biocyte laboratories are located and Ambrose is staying. As Ethan stakes out Biocyte, Nyah gets close to Ambrose and begins to learn about the Chimera virus. At a horse racing event, Ambrose meets with Biocyte's CEO, John C. McCloy. He shows McCloy a video of Chimera affecting one of Nekhorvich's colleagues. He then blackmails McCloy into cooperating with him. Nyah steals video footage and transfers it to Ethan. Ethan's team learn that Chimera has a 20-hour dormant period before it causes death by mass destruction of the victim's red blood cells. Bellerophon can only save the victim if used within that 20-hour window.
The IMF team kidnaps McCloy to force him to give up Bellerophon. However, they learn that the only samples of Bellerophon were taken by Nekhorvich, and are now in Ambrose's hands. Ambrose has the cure, but does not have the virus (which Nekhorvich injected himself with). As a result, Ambrose forced McCloy to exchange a sample of the virus for a sample of Bellerophon. The team break into Biocyte to destroy the virus. Ambrose, posing as Ethan, tricks Nyah into revealing his plan. Ambrose captures Nyah and raids Biocyte to secure the virus. Ethan is able to destroy all but one sample of the virus before Ambrose intervenes, and a firefight ensues. Ambrose orders Nyah to retrieve the last sample of Chimera. She injects herself with it, preventing Ambrose from simply killing her to get it. Ambrose takes Nyah away, and Ethan escapes from the laboratory.
Ambrose lets Nyah wander the streets of Sydney in a daze, intending to start a pandemic. He offers to sell Bellerophon to McCloy in exchange for stock options, to make him the majority shareholder. He predicts that the price of Biocyte's stock will skyrocket due to demand for Bellerophon after the Chimera outbreak. Ethan infiltrates the meeting and steals the remaining samples of Bellerophon. While Ethan is pursued by Ambrose, Luther and Billy locate Nyah, who has wandered to a cliff side, intent on killing herself to prevent Chimera from spreading. Ethan eventually gains the upper hand over Ambrose and kills him. With little time left on the 20-hour countdown, Luther reaches Ethan, takes Bellerophon and injects Nyah with it. IMF clears Nyah's criminal record and Ethan starts his vacation with her in Sydney.

Question: Who is pretending to be Ethan?
Answer: Rogue IMF agent Sean Ambrose
Alternative answer: Ambrose.