A subplot is a secondary plot strand 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.

In William Shakespeare's 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 his plot and Henry's do not mix. Even though they may be thematically connected, they are not connected in action.

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. When, as in Aleksandr Solzhenitsyn's Cancer Ward, about a group of patients at that ward, no one character's story clearly predominates, 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.