This very short story was part of the linking process in the MCU released in 2013. It is a lead-up to Captain America: The Winter Soldier.
It starts with Peggy Carter talking to Steve as he takes the nose-dive into the Arctic. Afterwards, she has been relegated to a desk job, doing the heavy lifting while the 'boys club' of Agent Flynn and his cronies go out on the exciting missions. Left behind to just 'tidy up a few reports', Peggy is the only agent present when an emergency alert comes in on a high-profile target called Zodiac. Peggy jots down the details, then crosses out the "3-5 agents" recommendation, and heads off to do some real clean-up herself.
Approaching the enemy's front operation, she is spotted, and patronized, by a couple of goons, who are rapidly left unconscious in her wake. The Third of the lookouts leads her, as she has him at gunpoint, into the back, and provides basic intel, including the presence of four guards still, himself included. Peggy 'persuades' him to scream for help, then very efficiently dispatches the three armed men who respond. The fourth, however, retreated into a locked room, presumably with the package. Peggy proceeds to shoot a hole in the metal door with carefully placed rounds, then toss in a smoke grenade, neutralizing the guard. Having slipped on a gas mask, she steps through the door and retrieves the key vial, with its Zodiac insignia, from a locked case in the room. As she stands to depart, however, she runs into a final, gasmasked, guard of massive proportions, who proceeds to strip off her mask and choke her. Flailing, she extracts a small hidden blade from the briefcase she brought with her, and stabs the giant, forcing him to release her. She then kicks him into oblivion, and leaves, pausing to warn the still-handcuffed first guard she brought in to learn to count.
The next day, dressed in a far more eye-catching ensemble, Peggy returns to work, only to be berated by Flynn for daring to ignore protocol. He informs her that only her status as "Captain America's old flame" has got her even this pity position, and intimates that he won't tolerate any further insubordination. He's interrupted by the alert signal, and goes to answer his direct line. The speaker instructs Flynn to identify himself, then tells him that it is Howard Stark. Mr. Stark wants to talk about Agent Carter's actions the previous night, but when Flynn assures him she will be disciplined, the billionaire tells the agent that Carter is needed in Washington, with him (Stark), to head up S.H.I.E.L.D.. He instructs Flynn to inform Carter that it is his honor to bring her this news, and the agent complies. When Flynn offers to help Carter move her personal possessions, however, she tells him, with considerable relish, that "as has always been the case, I don't require your help.". She then heads off to pastures new.
The inter-credit sequence is a short exchange between Howard and Dum-Dum Dugan, discovering the joys of bikinis - on women.