Citadel is downloadable content (DLC) and the final plot-oriented expansion pack to Mass Effect 3. It is set entirely within the Citadel, a central galactic hub that plays a key role throughout the series, and is built like a person sub-quest involving primarily Commander Shepard, the game's protagonist, not connected directly to the overarching story concerning the battle against the Reapers. The sub-plot begins when Shepard navigates the spaceship Normandy to the Citadel, and somebody attempts to kill him or her. Shepard decides to explore the Citadel and investigate the matter.
This expansion adds several new areas within the Citadel, such as a casino, the castle Arcade, and Shepard's personal apartment. It heavily focuses on interaction with Shepard's companions, including those not featured in the main game and only appearing in the predecessors, provided they were alive if the player imported a save file from a previous game: Urdnot Wrex, Miranda Lawson, Jacob Taylor, Jack, Grunt, Samara, Zaeed Massani, and Kasumi Goto. Shepard can throw a party in his apartment and the player can choose among various dialogues and interaction possibilities between the companions.
Citadel contains seventeen missions and also adds two new weapons, three armor sets, seven weapon mods, and two war assets that can be carried over to the main game.