Pioneering science-fiction writer H.G. Wells popularized time travel as a fictional plot device with his 1895 novel, 'The Time Machine.' Wells' novel contained the plot and character elements that would appear and reappear in countless science-fiction novels, novellas, short stories, and, later, in film and television. Time travel became a go-to plot device on several '60s TV shows, including 'The Twilight Zone,''The Outer Limits,' the aptly named 'The Time Tunnel,' and 'Star Trek: The Original Series.'

This Cinematical Seven, however, will focus primarily on key films in the time travel genre, beginning with the 1960 release of über-producer George Pal's adaptation of Wells' novel, stopping along the way to briefly discuss everyone's favorite time-travel comedy-adventure, 'Back to the Future,' multiple entries in the 'Terminator' and 'Star Trek' film franchises and ending (for now anyway) with Tony Scott's 'Déjà vu.'
Despite time travel's popularity as a plot device, it frequently involves, explicitly or implicitly, a paradox (or two or three). Some (most?) readers might be familiar with the "grandfather paradox," a paradox involving the following scenario: What if you could go back in time and stop your grandfather, by violence or other means, from meeting and marrying your grandmother. If you succeed, you negate your own existence. How then were you able to travel back in time? Similarly, traveling back in time to change present or later events, usually major, world-level ones, can create a similar problem for the time traveler. Of the films discussed below, only one explicitly covers this paradox, 'Back to the Future.'

[As always, spoiler alert is in full effect.]

The paradox issue, at least where the grandfather paradox is involved, can be ameliorated or eliminated by treating time not as we normally experience it, always moving forward in a straight line, but either as a closed loop with the time traveler's trip to the past necessary to sustain and maintain present and future events or, as 'Source Code,' opening this weekend, suggests, an open loop (or no loop).

1. 'The Time Machine' (1960) and (2002)*: Wells' novel caused little consternation or frustration on its release more 116 years ago, most likely because Wells' unnamed time traveler and the titular time machine don't journey into the past, but 800,000 years into the future. There, like many future-oriented time travelers, Wells' traveler discovered a dystopia, one where the passive Eloi lived above ground, their wants provided by the underground dwelling, cannibalistic Morlocks. As always, Wells' intended to critique his British contemporaries, but now as in 1960, viewers identify with the leisure-loving Eloi, not the industrious Morlocks. Wells' traveler journeys only once back to the 19th century, sharing his fantastical story with several colleagues before returning to the future in his time machine to rebuild Western civilization.

2. The 'Terminator' franchise, such as it is, can be confounding to new viewers, partly because of the James Cameron's non-involvement with the third and fourth films in the series and partly because Cameron, in an optimistic mood, shifted the treatment of time travel between 'The Terminator' (1984) and 'Terminator 2: Judgment Day' (1991).

(a) In the first, a killer cyborg from the future travels back in time (to 1984) to kill the future savior's mother before she gives birth. That savior, John Connor, sends a human warrior, Kyle Reese, to stop the cyborg. Connor, however, knows something Reese doesn't: Reese is his father. In Cameron's take, the future is just as set and unchangeable as the past. Treated as a closed loop time-travel film, 'The Terminator' ultimately makes sense.

(b) For 'Terminator 2: Judgment 2,' however, Cameron switched from closed loop to open loop time travel. The future can be changed, the nuclear apocalypse can be avoided, Skynet, the AI defense network that starts nuclear Armageddon, never becomes sentient. A new, still uncertain future awaits the survivors at the end of 'Terminator 2: Judgment Day,' a future that exists, more rather than less, in a parallel, branching reality.

(c) With Cameron gone as writer-producer-director, new producers took over the 'Terminator' franchise. Seeing more profit (rather than sense), they overwrote 'Terminator 2's' optimistic ending, restoring the first film's "war of the machines" against humanity. As 'Terminator 3: Rise of the Machines' ends in a nuclear Armageddon, moviegoers learned that Judgment Day could be temporarily, not permanently, postponed, meaning we're back to a semi-closed loop, a central, immutable timeline.

(d) Set post-Judgment Day, 'Terminator: Salvation' didn't involve time travel per se, though it feinted in that direction through the presence of a time-lost character (he's not, he just can't remember what happened to him several decades earlier, why he's still alive, unchanged, and why he seems to be super-strong). Presumably time travel would make an appearance in a fifth entry (unlikely given 'Terminator: Salvation's' lackluster box-office performance).

3. The 'Back to the Future' trilogy presents an interesting, if no less convoluted, set of conundrums (most of which are best left to a standalone essay on the subject), but 'Back to the Future' is far too important not to discuss in a time travel piece, especially list-centered pieces. Moviegoers' first encounter with hero-protagonist Marty McFly took them from contemporary (1985) suburban America to 1955 (still suburban America). Marty faces the grandfather paradox, taken one step closer (from grandfather to father), when he inadvertently comes between his teenage parents romance. In a wicked Oedipal twist, Marty's mother falls for him.

While fending off his mother's romantic advances, he has to bring them together or his future self disappears (or so we're led to believe). Marty's interference, however, leads to a better, brighter present for his parents. Complications from Marty's initial actions, however, reverberate across two sequels, one set primarily in 1955 again with Marty trying to fix a future-changing oversight and the other in the 19th century as Marty tries to save his mentor from a violent death. If there's only one timeline, as 'Back to the Future' and its sequels suggest, then the past is malleable as is the present and future.

4. 'Star Trek IV: The Voyage Home,''Star Trek VIII: First Contact' and 'Star Trek XI' (reboot): On television, 'Star Trek: The Original Series,' took several trips into the past, each time forcing the crew of the U.S.S. Enterprise to set things right, at least according to extant historical records. The past could be changed, but only with deleterious consequences for the crew of the Enterprise and everyone else in their future. Not surprisingly, the Enterprise's big-screen adventures inevitably involved time travel.

(a) In the pro-environmental 'Star Trek IV: The Voyage Home' (1987), Captain Kirk and crew travel to present-day San Francisco to kidnap whales and bring them back to the 23rd century. Without the whales, specifically whale song, an alien probe will destroy the earth. Nothing too complicated, time travel wise, except a marine biologist from our present who decides to travel back to the future. Presumably, she had zero effect on late 20th-century Earth, otherwise Kirk and his crew would have encountered a different future when they returned.

(b) With the original crew officially retired from cinematic adventures, 'Star Trek: The Next Generation' made the jump to the big screen with 'Star Trek: Generations,' but didn't go sans Kirk and crew until the next entry, 'Star Trek VIII: First Contact." In one of the most convoluted and, therefore conundrum-fraught entries in the 'Star Trek' canon, Captain Picard travels back to the 22nd century to stop the Borg, a hive-like race of cyborgs, from rewriting the future in their favor. 'First Contact's' writers gave themselves an out for the Enterprise's continued existence by having one character throw out the phrase "temporal wake" (a meaningless metaphor if there ever was one) as a catch-all explanation. Why and how the Enterprise and the Borg don't appear simultaneously time wise was left for audiences to ponder, something our next entry did as well. Ultimately, however, Captain Picard and his crew restore the central timeline (and continuity).

(c) For J.J. Abrams' reboot of the 'Star Trek' franchise, he explicitly chose to create a new, parallel timeline, leaving, presumably the more familiar timeline from 'Star Trek: The Original Series' on intact. Abrams' used Leonard Nimoy's Spock as a transitional character, traveling from the central timeline to the past and, through his presence and the presence of rogue Romulans, an all-new, parallel reality or timeline, allowing him to pay homage to classic Trek while simultaneously giving him the freedom to take the rebooted Trek in new, unfamiliar directions. A new timeline, however, didn't explain why two starships entering a black hole (hello time travel device) enter the past 25 years apart or why a single event, an encounter between the rogue Romulans and a Starfleet ship briefly piloted by Kirk's father would significantly alter major and minor events, even those seemingly unrelated. Maybe Abrams knows the answer to that question (but he's not telling).

4. 'La Jetée' (1962) / '12 Monkeys' (1995): French filmmaker Chris Marker's 28-minute film contains more thematic heft and visual poetry than all but a select few films. On the surface, Marker's film follows another unnamed time traveler, sent back to the present in a desperate attempt to save the future from post-apocalyptic ruin. The Powers-That-Be intend to send the time traveler into their future to seek aid from their descendants, but first send him back in time to a pre-apocalypse Paris. Chosen for his powers of recall, specifically an event from his childhood where he saw a man murdered as a beautiful woman looked at a local airport. If you're familiar with Terry Gilliam's '12 Monkeys,' a semi-remake of 'La Jettee,' then you know the identity of the murdered man, taking us back into closed loop territory (i.e., he always had to go back in time, to both witness and experience his own murder).

5. 'The Time Travelers' (1964): If you're unfamiliar with 'The Time Travelers,' you're not alone. A low-budget, mostly derivative programmer, 'The Time Travelers' centers on a group of time-traveling scientists who open a "time window" into the future where, surprise, surprise, a post-apocalyptic Earth awaits them. Flipping 'The Time Machine' around, above-ground dwellers have devolved into Neolithic cavemen, while the non-irradiated survivors live underground. The "time window" proves to be a time portal, allowing access to the future by simply stepping through to the other side. Although 'The Time Travelers' doesn't sound memorable, anyone who's seen it knows why it makes this list: the mind-bending ending where the central characters find themselves in a temporal loop, doomed, apparently to repeat the previous events ad infinitum (an idea 'Star Trek: The Next Generation' would pick up on two decades later).

6. 'Donnie Darko' (2001): Richard Kelly's first film as writer-director, 'Donnie Darko,' a box-office failure on its release, a cult favorite months later, bends and twists more than time. It also bends and twists genres (e.g., thriller, psychological drama, teen coming-of-age, suburban satire, science fiction) before allowing the title character to break a psychology- and personality-distorting temporal loop, but only by personally sacrificing himself and his future to save his mother and sister. The time loop closes, erasing evidence of Donnie's sacrifice and, with it, any paradoxes that might have otherwise arisen.

7. 'Time After Time' (1978): Nicholas Meyer's first foray into time travel (Meyer directed 'Star Trek II: The Wrath of Khan' four years later and co-wrote 'Star Trek IV: The Voyage Home' in 1987) turns writer H.G. Wells into a time traveling hero who pursues serial killer Jack the Ripper to contemporary San Francisco. Action, adventure, and romance take precedence over any time travel paradox, helped in large part by the contemporary setting (once in contemporary San Francisco, the characters stay there). It's the "future," after all. With history silent on the Ripper's eventual fate (the real Jack the Ripper was never captured), Meyer had the relative freedom to take his underseen, underappreciated film in any direction he wanted.

Honorable Mentions: 'Frequency' (son bonds with late father over time-traveling radio waves), 'Time Bandits' (preteen boy time-hops with a rogue band of little people, avoiding David Warner's imperious Satan), 'Primer' (a cautionary tale wrapped around a time-travel narrative wrapped around a Moebius strip), 'Groundhog Day' (a self-obsessed weatherman played by Bill Murray re-experiences the exact same day countless times, becoming a better man in the process), 'Time Cop' (Jean-Claude Van Damme as a time-traveling cop pursuing a power-obsessed senator played by the late Ron Silver), 'Déjà vu' (Denzel Washington's hero-cop rewrites the past, stops a terrorist attack, and saves the girl, but only one Denzel can exist in the newly rewritten past), and many, many others.