In my science fiction class, we broke the course down into three different plot types for the genre: "fantastic voyage", "aliens", and dystopia (although there probably are others). What I'm interested here is the first one, "fantastic voyages" (amusingly, despite the name, the movie Fantastic Voyage was not screened in the course), which are usually based around some sort of adventure or journey. Many space exploration type stories such as 2001: A Space Odyssey or Forbidden Planet would qualify, as would stories involving journeys to other fantastic destinations, such as the center of the earth (any film version of Journey to the Center of the Earth), or underwater (20,000 Leagues Under the Sea, The Abyss). What I am interested in looking at here, however, is one particular type of "fantastic voyage", specifically a voyage through time.
Time travel is something that has long been associated with the science fiction genre. In fact it arguably goes back at least as far as the genre's beginning with a novel by H.G. Wells (who along with Jules Verne was one of it's founding figures) simply called The Time Machine. Heck, even before that, Charles Dickens' A Christmas Carol had a strange (although supernaturally induced) form of time travel. There may be others I'm missing, but Wells was the first to explore the idea of a machine, produced by scientific means that could allow one to travel back and forth in time. At the time, Wells used his concept more to provide a social commentary in his vision of the future, but little did he know that he opened a door for many science fiction writers to come.
In any medium, time travel has also been around for a long time. There are a number of television shows that center around this concept, such as Doctor Who or Quantum Leap, and plenty of movies that find interesting ways to use it to different effects. Time travel is something that can take one idea and run with it in so many different ways, but what makes this idea of travelling back and forth through time so interesting?
For one thing, the concept of time travel isn't even strictly confined to the science fiction genre. In fact, in some cases the only "science fiction" aspect of the story is simply in how the character is explained to arrive in another point in time. Technically speaking there does not have to be a scientific explanation, as depending on the story the cause could be something supernatural or in some cases may not even need to be explained at all (they don't exactly offer up a scientific explanation for that time travelling car in Midnight in Paris, and the main character often seems to spontaneously return to his own time while just walking down the street).
That said, when it is brought up in science fiction, there are a variety of ways that can be done and to different effects. Both adaptations of The Time Machine simply involve a character being inspired to build an apparatus that is capable of travelling through time. The much better 1960 version has a discussion between the main character and several friends in which he attempts to explain time as "the fourth dimension" (after length, width, and thickness/breadth) before he travels into the future. A more famous example would be the time travelling DeLorean in Back to the Future. On the other hand, a movie like The Time Traveller's Wife bypasses the need for a machine at all, instead explaining it to be a genetic disorder.
Now there's a few different ways time travel can be used to great effect. The most obvious method is to explore the paradoxes that could ensue as a result. In Back to the Future Marty McFly ends up in the awkward situation where he accidentally undoes the event that led to his parents falling in love, and has to find a way to get them together to restore the timeline so that he can exist. The sequels in turn have Marty and Doc Brown realizing the dangers of time travel as further padoxes occur. Marty also just narrowly avoids becoming part of an "Oedipus Paradox", as he could have restored the timeline by staying with his mother and thus becoming his own father.
Another interesting form of paradox is the time loop. This is a weird form of time travel wherein events from the present manage to loop back into the past in a mind-boggling sense. Basically, this works under a different line of thinking to Back to the Future suggesting that time (at least everything in the past) is more or less fixed. If you built a time machine and went back in time, then you must have already arrived in the past, which therefore means that anything you do while your in the past has already happened in the present. In other words, travelling back in time to the past causes things to happen as they did in the present.
One of the most clear-cut examples would be The Terminator (though the sequel suggests at least some alteration is possible). The massive computer network Skynet sends a "terminator" (a killer robot designed to appear human) in order to kill Sarah Connor, the idea being to prevent her son from being born so he can't organize a rebellion against the machines. What ends up happening is that the resistance manages to send a guy back in time as well to protect her, and he explains everything that happens in the future. Eventually the two of them have sex and she becomes pregnant. In short, the effort by the machines to kill Sarah Connor was precisely what allowed her son to be born and thus the resistance to exist.
Twelve Monkeys and by extension the original short film La Jetée which inspired it, both made a twist ending out of this kind of paradox, wherein the protagonist unknowingly witnesses something that happens to a future version of himself.
This kind of premise can also lend itself to a few different kinds of films. In some ways it is a really good outlet for comedy in a fish-out-of-water sense, where the humor comes largely from the time traveler's unfamiliarity with wherever he or she is visiting. Back to the Future is a great example, where in addition to the paradoxes, a lot of humor comes from Marty not understanding how society worked in the 1950's or taking advantage of their lack of knowledge of 1980's pop culture (after all, in the present Marty could never get away with putting on a HAZMAT suit and claiming to be "Darth Vader from the Planet Vulcan").
Probably one of the best cases would be Bill and Ted's Excellent Adventure, which manages to take this style in two separate directions. First, we have two 1980's rock and roll fans travelling to various points in history and their interactions with people at each time. Hilarity ensues as they comment on and fail to understand the way things worked in the various time periods they visit. Then it goes the other way, when Bill and Ted return to the present, accompanied by a large group of historical figures. Bill and Ted are now in an environment familiar to them and the comedy instead comes from the historical figures like Abraham Lincol, Sigmund Freud, Ludwig Van Beethoven, Napoleon, Ghenkis Khan, Joan of Arc, Billy the Kid, and Socrates being unfamiliar with life in 1980's California.
On the other hand, time travel can also be used for a dramatic narrative. The Twelve Monkeys has the characters experiences with time travel lead him to gradually question his sanity. Bruce Willis's character of Cole even explains that he doesn't think the human mind can handle it (assuming of course the scenes set in the "future" are not merely delusions). The Time Traveller's Wife also uses it as the driving force of the relationship between its leads. It does touch on some strange paradoxes, such as the fact that Henry first encounters Claire at a point in her timeline after she first encounters him. Most of the movie emphasizes the struggle these two characters experience as they try to remain together despite the uncontrollable genetic problem that causes Henry to keep jumping back and forth through time at random.