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A story arc is a storyline that continues for more than one individual unit of a source canon, for instance, for several episodes of a television show, several issues of a comic book, or several books in a series. Developments in the story arc may occur as subplots or B-plots, culminating at the end of the season or the conclusion of the series. They are thus distinguished from serials, as in radio theater, where the main story is broken into daily episodes. Some serial dramas did have subplots, often more than one.
Story arcs are common in television series today. For example, the first season of Veronica Mars features several season-long story arcs, such as Veronica's investigation into Lilly Kane's murder, as well as several shorter arcs, such as Keith Mars and Alicia Fennel's romantic interest in each other, and the issue of Veronica's paternity. The Harry Potter books feature several multi-book arcs; for example, Harry's investigation of Severus Snape's true loyalties.
With the exception of soap operas, story arcs were generally not allowed in television drama until the early 1970s. Part of the reason for this was a belief that viewers needed to be able to tune in at any point in the season and be able to tell what was going on. This was especially important when the show went into syndication, when it might reach many more viewers than while it was on the network.
Almost nothing was serialized back then—there were no “story arcs” as we know them now. Personal and professional traumas begin in the first act of an episode and are wrapped up in the last one; come the next episode, it’s as if they never happened. McGarrett is kidnapped and tortured by his nemesis, Wo Fat, but in the following episode, he carries on as if the horrors had never happened. We see McGarrett in love, only to have his ladyfriend murdered halfway through the episode, but neither the woman nor the murder is ever mentioned again. Even the death of beloved 10-season character Chin Ho Kelly takes place in a single episode—and about 15 minutes into the episode at that, as weak, undignified, and unsatisfying an adios as has ever been visited on a well-established TV character. Today, any one of those stories would play out over several episodes, or maybe even a whole season. The only exception to the no-carryover rule involved multi-part episodes.
- Individual characters may be said to have character arcs. These may be carefully planned by TPTB, or divined from canon evidence by fans after the fact. (See Watsonian vs. Doylist).
- Particularly mysterious or complex story arcs, especially in science-fiction shows, may be called mytharcs. (An episode of an arc-heavy show which does not include any references to the overarching storyline is called a standalone; see also Monster of the Week.)
- In anime fandom, an anime which quickly outpaces its source manga and needs to slow down a while (to wait for more canon developments) may resort to a filler arc.
- Comics fandom also uses the term story arc to describe a universe-wide plot development, which generally affects many individual comics titles, tying them together in a sort of multi-book crossover. 
- Some people count I Love Lucy as having some story arcs: Lucy's pregnancy and the birth of Little Ricky, the Ricardos' and Mertzes' trips to Hollywood (which stretched over two seasons) and Europe, and their eventual move to Westport, Connecticut.
- Another early example of story arcs on primetime network television outside a soap opera was on Barney Miller in the mid-1970s to early 1980s. A series of subplots revealed Barney's marital problems, Inspector Luger's search for a wife, and especially the evolution of Ron Harris' writing career. Several episodes were devoted to the story of gay Ofc. Zatelli's coming out; series creator Danny Arnold worked closely with the National Gay Task Force to develop the Zatelli storyline.
- X-Files fandom, which coined the term "mytharc."
- Babylon 5 was originally conceived as one five-year long story arc.
- DC Comics' 1992 arc The Death of Superman, which required fans to read multiple different titles in order to follow the entire story (including Action Comics, Superman, Superman: The Man of Steel, and Adventures of Superman.)
- Many fans may have initially heard the term "story arc" in reference to the disastrous third season of Lois and Clark. After a huge publicity buildup for the "wedding episode", including heart-shaped invitations, mention on ABC News and even other networks, and airing close to Valentine's Day, viewers were shocked and angered when the woman Clark married turned out to be a frog-eating clone, while the real Lois had amnesia. The show's producers had intended a real wedding, but the network heads forced the change fearing that people would stop watching the show after they finally got together.
- Wiseguy's "Steelgrave arc," practically its own mini-fandom.