Basically, if time travel exists, then its use is required.
Say you've ended up on a different planet (Hey, we have time machines here!) and, being adept at saving the day, you try to get about your heroics as normal. But, the overwhelming monsters won't stop their genocidal plan unless they have a certain MacGuffin — great, we'll just get it! Except, we only know where it is on certain dates, and today isn't one of them. Looks like you've got a long fight on your hands... wait, where's that time machine.
This isn't the only scenario, you might need to hop back a few times to start the fight right (like Save Scumming in a video game) or suddenly need to throw something into the end of the universe. Or you might just need to be hopped out of there quickly at the end. It might not even be a fight, but any plot done in a setting with time travel — regardless of how it would be done anywhere else — absolutely needs the use of time travel. In some cases, even the romantic and/or comedic side plots will also hit an issue that requires time travel, or in some other way actively incorporate the ability.
Sometimes done as part of The Plan, when the idea that "we have time travel, we should use it" is invoked.
This also covers where Inter-Dimensional travel is possible, but time is more common.
Examples:
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Anime & Manga
- Dragon Ball Z: In the Cell Saga, Bulma, Gohan and Future Trunks discover another version of Trunks' time machine, which was later revealed to have been stolen by Cell when he killed his timeline's Trunks. Flash forward to Dragon Ball Super's Black Goku Saga and it's revealed that Bulma kept the time machine and she pulls it out when Black Goku destroys' Trunks' time machine with the intent on stranding him in the past.
- Doraemon himself comes from the 22nd century with a time machine to help Nobita in his school days. At least in some episodes/chapters, Doraemon and/or Nobita has to solve the problem of the day by going to the past or future with said time machine.
Films — Live-Action
- Back to the Future: Outside of some vague Foreshadowing and the film's own title, there's zero indication that it's about time travel. Then Marty meets Doc at a parking lot to film an ultimately successful test of his latest invention: the DeLorean Time Machine. Per this trope, Marty accidentally uses it trying to escape from the Libyans a few minutes after the test.
- Marvel Cinematic Universe: In Ant-Man and the Wasp, it is mentioned that the Quantum Realm has "time vortexes". In Avengers: Endgame, Scott Lang and the remaining Avengers use these properties to travel in time to try gathering the Infinity Stones and undo Thanos's snap from Avengers: Infinity War, which wiped out half of life in the universe.
- Galaxy Quest: The "Omega 13" device is mentioned at the beginning of the film, but no-one knows what it actually does. In the middle of the film, the protagonists pass by the real one the Thermians made on their way through the ship, and the uber-fan guiding them speculates that the Omega 13 allows its user to go back thirteen seconds into the past, just enough time to correct a mistake — though he also brings up the possibility that it would destroy the universe in 13 seconds, so it's brushed off as irrelevant. Then, in a moment of desperation at the climax, they activate the Omega 13 for real, and it does indeed send them back thirteen crucial seconds.
- Inception: The "limbo" subconscious level of dreamworlds (if counting as a functional alternate dimension) that is brought up quite early on is exploited come the climax of the film, becoming a dramatic device with Cobb and Ariadne deliberately killing themselves to get there to save the lives of Saito and Fischer. It also gives Cobb closure with Mal.
Literature
- The Hitchhiker's Guide to the Galaxy: Time travel, acknowledged but never needed for a while in the stories, is used to escape from mice who want to dissect Arthur's brain. As a subversion, the mice then have the option to create another biological super computer to find out what they wanted to know, but which would take 10 million years to process it — instead of making it and then travelling in time for the answer, they decide that it's too long to wait and falsify data.
- Harry Potter and the Prisoner of Azkaban: When it is revealed that time-turners exist (upon Hermione taking all the possible electives in her third year), it suddenly becomes necessary to use to disguise the group's actions, and to deliver Harry to the lake, to save himself from the Dementors, though he'd thought it was his father.
- Time After Time by Karl Alexander is a story about Jack the Ripper doing, well, what he is known for. By coincidence, he ends up at a showcase which features H. G. Wells showing off his actually-real time machine. The police arrive here to arrest "Stevenson" (Jack), so he steals the time machine to escape.
- Ishmael (1985) is a Star Trek-verse novel in which Spock travels back in time to explore the set of "Here Come the Brides" in Seattle, where he discovers that the Klingon are plotting to use time travel to kill one of his own ancestors — not to prevent him from being born, but because they were instrumental in another war. In addition, Spock is relatively amnesiac for most of the story and may have actually got another set of his predecessors together.
Live-Action TV
- Doctor Who: Most relevant with the Gallifreyan council: whenever they pop up, they want to use time travel for something. Most pertinent example: The Time War is sort of the big underlying event of the series, and in "Genesis of the Daleks", the council force the Fourth Doctor to go back and destroy the Daleks before they started the War. When he has the chance he chooses not to, and this is what makes the Daleks start it.
- Arrowverse:
- The earlier The Flash/Supergirl (2015) crossover showed that these heroes were getting better at inter-dimensional travel. The crossover opens with a need to go find Supergirl on Earth-38 because her alien expertise is needed to help fight the Dominators.
- Legends of Tomorrow: From the pilot:
- Rip needs to travel back in time to stop Savage from turning to world into dystopia. It's also as a result of Rip trying to stop him that Savage hunts down Rip's family and kill them when he does take over.
- Subverted when they're in the Seventies and need to use an alpha particle tracker so they can find a piece of Ray's suit that they accidentally left behind, and the trackers aren't invented until the 21st century. First, they don't think to go back in time a little bit to grab the piece of suit where it fell before someone else did, and second, they didn't hop forward in time to get a tracker, they went to find the pioneer of alpha particle research and try to steal his prototype.
- Andromeda: Harper has to use time travel/warping several times, including to cure himself of an infection by altering time-space.
- Castle: One episode has an assassin from the future come to kill a scientist before he can invent force fields. Even though he is running around the present, Castle still recruits another time-traveler to stop him.
Theatre
- Harry Potter and the Cursed Child: Time turners are the driving force of the plot and alternative timeline.
Video Games
- BlazBlue: Nu-13 dragging Ragna into the Cauldron causes a "Groundhog Day" Loop where the time loops to where the two falls into, i.e 100 years before. Then, in the third game, some days after the time loop is brokern, a Time Travel with the Cauldron is used more deliberately by Rachel carrying Ragna to about 94 years ago to do a timeline fix that "makes the history more defined" (long story).
- EarthBound (1994): The Meteor that starts off the adventure by bringing Buzz Buzz to Onett is used much, much later to gain the material used to go back in time to defeat Giygas.
- Empire Earth: The Russian campaign features a Chinese Time Travel attempt being foiled. Two missions later, renegades from Novaya Russia ally with the US to go back in time in order to prevent Novaya Russia from becoming the totalitarian nightmare, essentially reliving the first Russian level from the other side. The ending cuts just before we see the effects this has had on the future, (even though the sequel features the same faction, it has no impact on the plot).
- Bill And Teds Excellent Adventure: Set after the film, Bill and Ted's girlfriends are kidnapped, and the only way to rescue them is to collect music from different time periods.
- Cryostasis Sleep Of Reason: The protagonist player has the power to enter people's minds and change their past actions. They may become useful in getting them out of your way.
Web Comics
- 8-Bit Theater: The creating of a Stable Time Loop is what causes Onion Kid to become Sarda.
- Ansem Retort: The team time travel to find Jesus for the hangover cure when they get drunk.
- Done brilliantly in Casey and Andy: The Nineteenth Century White House Teleport-O-Mat was introduced as a one-off joke about one of Andy's obsessions. Much later, the protagonists use a proper time machine during a time travel arc; but once banished to back their own time without the machine, they use the former to get back into the fray.
Western Animation
- Steven Universe: The pilot episode has, after Steven's abuse of a time travel MacGuffin attracts the attention of a powerful alien, Garnet pull a rather out-of-character move by declaring "Steven, why are you such a buttface?" When the Crystal Gems are beaten, Steven remembers the insult and uses the MacGuffin (which is triggered by him coming up with the perfect comeback to an insult) to go back before the attack and save everyone. Garnet's reaction to the comeback implies she did this deliberately.
- Defied by the creators of Rick and Morty, where Rick has a box on his shelf with the text "Time Travel Stuff", but time travel is about the only sci-fi trope they haven't touched yet. Word of God said the box on the shelf is a Stealth Pun, indicating that all time travel stories are "shelved" for the series. (It's for this reason that Rick can't simply go back in time to when Mulan was running in theaters to try McDonald's Szechuan saucenote ). One scene has Rick grabbing a pizza they ordered from an alternate universe that's about a half hour ahead of ours, and he goes out of his way to point out this isn't technically time travel.
- In the TV Popeye cartoons from the Jack Kinney studio (Format Films), Professor O.G. Wottaschnozzle has a time machine with which he uses Popeye as its traveler. Popeye then becomes a component in world history (except for an episode where Olive Oyl accidentally got sent by the machine to a metallic planet in the far future).
