A recap sequence (or recap, often announced as "Previously on...") is a narrative device used by many television series to bring the viewer up to date with the current events of the stories' plot. It is usually a short (between 20 and 40 seconds) montage of important scenes cut directly from previous episodes, usually short bursts of dialogue, which serve to lay the background for the following episode.
These devices are predominantly seen on episodic series where every installment furthers the plot, but otherwise are used for plot arcs, that is, when an episode is going to pick up on a storyline initiated several episodes ago. A recap will almost always be present in an episode that picks up from a cliffhanger. Recap sequences are most prevalent in dramatic television series (though daytime serials are usually excluded) and reality programs, whereas situation comedies and other scripted genres typically use recap sequences only for two-part or in rare cases, multi-part episodes.
These montages are inserted as the first thing into an episode so that viewers who had not seen the previous episodes or who do not remember what happened can understand from where the current episode will begin, also so that the viewers may decide to catch up on missed episodes, usually buying the DVDs. They usually begin with a voiceover or subtitle proclaiming, "Previously on... [the show's name]" or "Last time on... [the show's name]". Many shows have begun to use a main character's voice for this voiceover rather than a neutral narrator (Chuck), and some series such as Boston Legal and Stargate Atlantis alternate which main character says "Previously on... [the show's name]".
On the Fox show 24, the main character, Jack Bauer's voiceover "Previously on 24..." is followed by scenes from prior episodes of the current season, not necessarily from the very last episode, that concern major plot points of the current episode. This was carried over into 24: The Game, where, once a player completes a mission, all cutscenes prior to that mission can be viewed in the manner of the TV show. The playable scenes are not shown, as the player is given the opportunity to replay them to get a higher score.