Margaret Carter was an agent of the British Secret Intelligence. In November 1940, she infiltrated Castle Kaufmann, the headquarters of the SS general Johann Schmidt, leader of HYDRA, Adolf Hitler's top secret science division. Disguised as a maid Eva, she freed the captured scientist Abraham Erskine and brought him into the Strategic Scientific Reserve.
Captain America: The First Avenger
Peggy served as a general in the American Army. She came across Steve Rogers, a wimpy young man with a great will to serve his country. This young man was nominated for "Project: Super Soldier". It was her duty to safely escort him to the laboratory. The experimentation had almost failed, though Steve persisted. After the great struggle to keep him from dying, the "Super Soldier" was created. Peggy began taking interest in Steve. Peggy soon developed a crush for Steve, though referred to it as "having faith in him". After a brutal fight to the death on a HYDRA plane, Johann Schmidt was finally defeated, along with his soldiers. Knowing that the plane Steve was on would cause a lot of trouble for America, he was left with one option; forcing the large plane to land on the waters of the Northern Atlantic Ocean. Peggy, knowing that this would result in his death, doesn't give up hope. She insists on the matter, implying that there would be time for them to sort the situation out. Steve claims that there isn't enough time, and proceeds with the landing.
Steve had survived, but in a quite unexpected matter. It appears that he had been frozen for over 70 years. The experimentations that were done on him in the past had allowed him to survive, and retain his youthful appearance. Her current fate is unknown at this point, though she is presumed dead. However, when Steve met Nick Fury in present-day Times Square, he mentioned the date he'd promised her almost seventy years ago.
Captain America: The Winter Soldier
Peggy has since been married and had children. She is ill, and Steve visits her in the hospital. Although she initially appears to recognise Steve, she slips back into a state where she believes to be seeing him again for the first time since he was lost. Shocked to see him alive, she laments, "It has been so long!". He merely says, "I couldn't leave my best girl. Not when she owes me a dance."