In fiction, a subplot is a secondary strand of the plot that is a supporting side story for any story or the main plot. Subplots may connect to main plots, in either time and place or in thematic significance. Subplots often involve supporting characters, those besides the protagonist or antagonist.

Subplots are distinguished from the main plot by taking up less of the action, having less significant events occur, with less impact on the 'world' of the work, and occurring to less important characters.

In screenwriting, a subplot is referred to as a "B story" or a "C story," etc.

In William Shakespeare's play Henry IV, Part II, the main plot concerns Henry's growth from "Hal" the prince to "Henry" the king and the reconquest of French territory. A subplot, however, concerns Falstaff's participation in the battles. Falstaff and Henry meet at several points, and Falstaff is a familiar of Henry's, but otherwise his plot and Henry's do not mix. Even though the plots may be thematically connected, they are not connected in action.

In William Shakespeare's play King Lear, the main plot describes how Lear disowns his faithful daughter Cordelia and divides his Kingdom between his treacherous older daughters Goneril and Regan. However, there is a subplot involving the Earl of Gloucester and his two sons, Edgar and the illegitimate Edmund. Edmund tricks Gloucester into thinking the faithful Edgar is plotting against him, causing Edgar to flee. This subplot reflects the main events of the plot, i.e. Fathers mistaking their good and bad children. However the events mingle, Goneril and Regan fall in love with Edmund after he becomes Earl, and in the rewrite of the Play by Nahum Tate, Edgar marries Cordelia at the end.

When no single story dominates a narrative, as in Aleksandr Solzhenitsyn's novel Cancer Ward, the plots will not be distinguished into the main plot and subplots. Because of their brevity, short stories and to a large extent, novellas, usually contain no subplot.

The video game Xenogears is famous for having fourteen different plotlines. Which ones are main plots and which are subplots can only be distinguished by the influence that the characters involved have over the game's setting and the other characters.