A subplot is a series of connected actions within a work of narrative that function separately from the main plot. Plot--the connection of events in a temporal or metaphorical line--is distinct from action (events themselves), and when a novel, play, narrative poem, or visual narrative has both a central plot and a second set of connected actions that is separate from that plot, it is said to have a subplot. Subplots may connect to main plots, but they cannot carry forward, strengthen, or explicate that main action.

In "Henry IV Part II," for example, 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.

A subplot is sometimes referred to as a "B story" or a "C story" and so on, with the "A story," being the main plot. Subplots often involve supporting characters.