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A contrived or arbitrary plot device may annoy or confuse the reader, causing a loss of the suspension of disbelief. However a well-crafted plot device, or one that emerges naturally from the setting or characters of the story, may be entirely accepted, or may even be unnoticed by the audience.
Many stories, especially in the fantasy genre, feature an object or objects with some great power. Often what drives the plot is the hero's need to find the object before the villain and use it for good rather than evil, or if the object has been broken by the villains, to retrieve each piece that must be gathered from each antagonist to restore it, or, if the object itself is evil, to destroy it. In some cases destroying the object will lead to the destruction of the villain. In the Indiana Jones film series, Jones is always on the hunt for some mystical artifact. In Raiders of the Lost Ark, he is trying to retrieve the Ark of the Covenant; in Indiana Jones and the Last Crusade, Jones is on a search for the Holy Grail. This plot device dates back to the medieval Arabian Nights tale of "The City of Brass", in which a group of travellers on an archaeological expedition journey across the Sahara to find a brass vessel that Solomon once used to trap a jinn.
Several books in the Harry Potter series orient around a certain object. In Harry Potter and the Philosopher's Stone, Harry believes there is a magical stone in Hogwarts with special powers. Lord Voldemort needs this stone to bring back his body, and Harry looks for the stone first to prevent Voldemort's return.
The One Ring from J. R. R. Tolkien's novel, The Lord of the Rings has been labeled a plot device, since the quest to destroy it drives the entire plot of the novel. However, as Nick Lowe puts it: "Tolkien, on the whole, gets away with the trick by minimizing the arbitrariness of the ring's plot-power and putting more stress than his imitators on the way the ring's power moulds the character of its wielder and vice-versa.".
A MacGuffin is a term popularized by film director Alfred Hitchcock, referring to a physical object (or character) which drives the actions of the characters as they search for it or try to obtain it, but whose actual nature is not important to the story. Another object would work just as well if the characters treated it with the same importance. Hitchcock said that "in a thriller the MacGuffin is usually 'the necklace'; in a spy story it is 'the papers'".
MacGuffins are frequently found in 'quest' fantasy stories as the magic artifact which the hero must recover in order to save his village, world, or family. Examples include the Labours of Hercules.
MacGuffins are sometimes referred to as "plot coupons" (especially if multiple ones are required) as the protagonist only needs to "collect enough plot coupons and trade them in for a dénouement". The term was coined by Nick Lowe.
The term deus ex machina is used to refer to a narrative ending in which an improbable event is used to resolve all problematic situations and bring the story to a (generally happy) conclusion.
The Latin phrase "deus ex machina" has its origins in the conventions of Greek tragedy, and refers to situations in which a mechane (crane) was used to lower actors playing a god or gods onto the stage at the end of a play.
The Greek tragedian Euripides is notorious for using this plot device as a means to resolve a hopeless situation. For example, in Euripides' play Alcestis, the eponymous heroine agrees to give up her own life to Death in exchange for sparing the life of her husband, Admetus. In doing so, however, she imposes upon him a series of extreme promises. Admetus is torn between choosing death or choosing to obey these unreasonable restrictions. In the end, though, Heracles shows up and seizes Alcestis from Death, restoring her to life and freeing Admetus from the promises. The first person known to have criticized the device was Aristotle in his Poetics, where he argued that the resolution of a plot must arise internally, following from previous action of the play.
A shoulder angel is a plot device used for either dramatic or humorous effect in animation and comic strips (and occasionally in live-action television). The angel represents conscience and is often accompanied by a shoulder devil representing temptation. They are handy for easily showing inner conflict of a character. Usually, the angel is depicted on (or hovering near) the right shoulder and the devil or demon on the left, as the left side traditionally represents dishonesty or impurity (see Negative associations of left-handedness in language).
Another form of plot device is the object, typically given to the protagonist shortly before, that allows them to escape from a situation that would be otherwise impossible. Nick Lowe coined the term 'plot voucher' for these ("This voucher valid for one  awkward scrape. Not transferable.") Examples of this might include the object given to a character which later stops or deflects an otherwise fatal bullet. Most of the devices given to James Bond by Q fall into this category.
Other plot devices are simply intended to get the protagonist to the next scene of the story. The enemy spy, who suddenly appears, defects, reveals the location of the secret headquarters, and is never heard of again, would be an extreme example. Without this 'device' the hero would never find the headquarters and be unable to reach the climactic scene; however the character becomes less of a plot device if the author gives them a back-story and a plausible motivation for defecting and makes them an interesting character in their own right.
Another kind is the device that overcomes some technical difficulty of the plot which is not important to the story but which needs to be explained. Devices for turning foreign (especially alien) languages into English, so that the audience and characters understand them are one of the most common.
Some video games rely heavily on plot devices; games are sometimes entirely centered around characters performing arbitrary tasks in order to win the game. Even well-plotted games sometimes involve the protagonist in a series of relatively unconnected and unjustified tasks.
Some other plot devices include: