Natalia Alianovna "Natasha" Romanoff was born in Stalingrad, a region in Russia, in the year 1984. Natasha was recruited by the main the security committee of the Soviet Union, the KGB, at a very young age. She endured her training at the Red Room Academy, a soviet facility used to train spies and assassins and was deemed the most talented spy and assassin to graduate from the academy.
Romanoff's skills and expertise were seen as a threat and triggered Nick Fury to send one of his best people from S.H.I.E.L.D to terminate her. That man was Clint Barton aka Hawk Eye. Clint pursued Natasha but not with intent to kill her. He recognized very quickly her potential and decided that she would be great as an agent at S.H.I.E.L.D.. Natasha is recruited by S.H.I.E.L.D. with the recommendation of Clint, and from then on they developed a strong friendship, eventually becoming best friends and going on numerous operations together.
Natasha's was assigned to keep an eye on Tony Stark, after he had signed over his entire company to his then assistant, Pepper Potts. Her mission was to evaluate Tony to see if he was fit for the Avengers Initiative. She arrived at the Stark Tower as an applicant to be his assistant, and was immediately granted the job after a display of unexpected combat skills that she used to take Happy Hogan down in the boxing ring, and after Tony learned that she came with a very impressive resume, including being able to converse in multiple languages.
After her mission with Tony was complete, Natasha continued going on other S.H.I.E.L.D. operations until she finally gets a call from Agent Coulson during her mission to take down an illegal weapons dealer named Georgi Luchkov, where she was instructed to bring Bruce Banner in as part of the Avengers Initiation. She tracks down Bruce and brings him in to help with locating the Tasseract, during which they were acquainted with Steve Rogers.
The Tasseract was in Loki's possession at the time, so after flying to Germany to capture him and on their way back, they are visited by Thor who made a dramatic entrance with a lightning storm, claiming that Loki is a prisoner of Asgard and left with him, not giving the rest of them time to react. Tony Stark aka Iron Man, then pursues Thor with Steve Rogers aka Captain America joining in, and they fight to retain possession of Loki and the Tasseract.
After that misunderstanding and much later, Natasha and the rest of Avengers assemble to fight and defeat Loki and the Chitauri Army. Natasha fights with the Avengers in every one of their battles until she finally sacrifices herself before the final battle against Thanos after the Snap, in order to retrieve the Soul Stone to be used by the Avengers to reverse the effect of the Snap.