Every Fugue has its own form. A form is created by alternations of expositions (sections with full statements of the subject) and episodes. As a fugue develops, it often modulates to several different closely related keys before returning to the tonic key. The first exposition usually has an short episode within it (after subject and answer) that harmonically prepares the next statement of the subject on the tonic. Sometimes (if the answer is in the dominant key) the function of the episode is to provide a modulation back to tonic. Most of the later episodes are longer (they greatly vary in length).
Example: Bach, WTC I, Fugue V in DM
This episode functions as a modulation from em back to the tonic DM.
These three measures are followed by a iii - vi - ii - V - I circle progression, in which the modulation to DMis fully established with a cadence.