The full story
A year after Captain America: The First Avenger, Peggy Carter works for the Strategic Scientific Reserve, where her boss, Agent John Flynn, keeps her compiling data and breaking codes while the men get the field cases. The SSR's priority is the Zodiac, a mysterious serum it has failed to recover for some time.
One night, alone in the office while the men are out together, Carter takes a call on the case line giving the Zodiac's location. Three to five agents are recommended. She goes by herself, fights her way past the guards, and comes back with the serum.
The next day Flynn reprimands her for skipping procedure, and dismisses her as Captain America's old flame, given the job out of pity. Before he can punish her, the case line rings again. It's Howard Stark, telling Flynn that Carter will co-head the newly founded S.H.I.E.L.D. In a mid-credits scene, Dum Dum Dugan is lounging poolside with Howard.