With World War II over, Peggy Carter left England and chose to live in the United States of America. She lived in an apartment with Colleen O'Brien and took turns sharing the bed; O'Brien slept there during the day, while Carter worked for the Strategic Scientific Reserve and Carter slept at night as O'Brien worked in a factory. One day, O'Brien returned from work feeling ill; Carter told her to enjoy sleeping, even suggesting that she take a sick day.
Meanwhile, Carter worked for the New York Bell Company Office branch of the SSR which was investigating Howard Stark and the cache of inventions stolen from him. Among the stolen weapons was a formula for an explosive called Nitramene; Carter, helping in the investigation, but on the behalf of Stark through their friendship, discovered that the Nitramene had been converted to an active bomb. Carter took the bomb from the nightclub La Martinique and took it home to the apartment to diffuse.
Peggy Carter returned to her apartment from La Martinique with the Nitramene to find her roommate Colleen O'Brien sick in bed. Carter entered the bathroom to prepare a solution of sodium hydrogen carbonate and acetate using a combination of baking soda and vinegar. Carter used the solution to successfully neutralize the Nitramene and poured a drink.
Carter heard a noise coming from the bedroom and went to investigate. Carter approached the bed and pulled back the cover to find O'Brien had been shot in the head. The Man in a Green Suit exited the closet behind Carter and took aim but Carter spotted him in the mirror and kicked him in the leg and grabbed his gun. The two struggled briefly over the weapon until Carter was able to knock it from his hand.
Green Suit threw Carter against the wall and Carter saw a peculiar scar on the man's throat. Carter put the man's hand on the burning stove and he picked up a knife from the sink. Carter dodged several knife attacks before throwing the man out the window. Carter ran to the window but the man was gone.
Not having many friends in New York City, Peggy Carter cried hard over the death of Colleen O'Brien. The next morning, Carter confided in Edwin Jarvis about how discouraged she felt; Jarvis advised her to continue her mission so that O'Brien's death would not be in vain.
Carter and Jarvis went to Anton Vanko to learn who would be able to convert the Nitramene formula to an active bomb. Vanko led them to the Roxxon Refinery and, ultimately, to a battle leading to its destruction.
Carter was homeless since Colleen O'Brien's Apartment was in O'Brien's name, without anyone knowing that Carter lived there. This helped her to avoid questions about O'Brien's murder, however. Jarvis gave Carter a place to sleep, before she eventually moved into the Griffith Hotel.