Chimera was a virus created by Biocyte Pharmaceuticals and its cure is Bellorophon. Biocyte intended to release the virus in the public worldwide so they can get massive profit from monopolizing its cure.
A Russian research scientist and Ethan's good friend, Dr. Vladimir Nekhorvich, infects himself with Chimera virus as he plans to go to the Center of Disease Control in Atlanta on a flight from Sydney within 20 hours of departure, before a former IMF agent, Sean Ambrose, boards his plane; Boeing 747, while disguising as Ethan Hunt, and kills him and made his escape from the plane with his right-hand man Hugh Stamp, along with two of his operatives; Wallis and Ulrich with Nekhorvich's bag.
At a horse racing event, Ambrose meets John C. McCloy, a British CEO of Biocyte, where Ethan and three members of his team; computer specialist Luther Stickell, British professional thief Nyah Nordoff-Hall and Australian pilot William "Billy" Baird, gather information on Biocyte and McCloy.
Conducting a raid on Biocyte's head office in Sydney, Ethan destroys all but one Chimera sample and Ambrose and his men engage Ethan in a firefight before ordering Nyah to pass him the last Chimera injection gun, but she injects herself with it. Ambrose lets Nyah run wild to spread the virus as Ethan, disguising as Hugh, gets the Bellorophon and Chimera before escaping Biocyte's island complex to cure Nyah after killing Ambrose for his crimes against humanity.