A bottle episode is a television episode which is designed to be produced at minimal cost, typically using already existing sets or sometimes by limiting the scenes to only one location. Although a bottle episode is cheap to produce, it would be a mistake to dismiss such episodes as filler. In fact, bottle episodes can be very important for plot and character development, with writers focusing on making the episode strong, since the audience does not have visual distractions to keep it entertained.
The term originates from Star Trek, where a number of episodes took the form of bottle episodes. The crew of the show came to refer to such episodes as “ship in a bottle” episodes, referencing the fact that they typically took place on board the Starship Enterprise, and the term "bottle episode" caught on more widely in the television industry. These episodes are also sometimes called “bottleneck episodes,” referencing the constricted budget available to the crew.
When a season of a television series is mapped out, bottle episodes are planned in. Typically, the producers have a limited budget to work with, and they want to use it effectively. Rather than producing a series of relatively mediocre episodes, producers often choose to dedicate more funds to certain episodes, pulling out all the stops to create a memorable and distinctive episode for the audience. For example, season openers and finales tend to have higher budgets, allowing the writers and crew greater leeway to play with in the hopes of drawing viewers in.
Some people mistakenly use the term “bottle episode” to refer specifically to an episode set in a single location, but the term can be used more generally to talk about episodes which only take place on existing sets, and the action may move around quite a bit. In the case of an episode shot in a single set, term “dog day afternoon” sometimes crops up, referring to the 1975 film Dog Day Afternoon, which is set solely in and around a bank.
In the case of a dog day afternoon episode, the writers may use an interesting gimmick to keep everyone in the scene, such as a locked door or a disaster which forces everyone to stay inside. In these scenarios, guest appearances are uncommon, allowing writers to focus solely on the familiar characters of the show. It is not uncommon for a bottle episode to be used to advance a particular point of plot, or to achieve a desired goal for a particular character. Among fans, such episodes are sometimes extraordinarily popular, despite the fact that they are less visually interesting than shows set on new, lavish sets or exotic locations.