Retroactive Preparation

A Time Travel trope, Retroactive Preparation is a twist on the You Already Changed the Past plot where the existence of a Stable Time Loop works to the advantage of the character's goals rather than thwarting them.

Let's say your favorite show is about to air, but you forgot to program your VCR or DVR to record it. You run up to your front door, discover it locked, and realize you don't have your keys. What do you do? Break a window? Bust down the door? Drop down the chimney? Watch it at a neighbor's house? Hurry! There's less than a minute left! Oh, only if you had a key hidden under the doormat or something!

But wait! You have the next best thing: A time machine!

If you have a time machine, then no matter what hurdle is set before you, you have all the time in the world to prepare for it after you've already overcome it, even if you are running on San Dimas Time. This rule mainly applies when the situation can be solved by having the right equipment at the right time, and the people in said situation are aware of the Stable Time Loop. It's like being Crazy Prepared, minus the foresight. In the example above, all you'd have to do is check the place where you might have hidden the key, then after watching your show, go back in time and hide the key there.

Of course, there's no need to actually show anyone setting it up, just the end results.

Compare Tricked-Out Time, the circumvention of the Stable Time Loop, and contrast You Already Changed the Past, the failed attempt to Set Right What Once Went Wrong. See also Temporal Paradox.

Anime and Manga

 * In The Disappearance of Haruhi Suzumiya, Kyon makes use of this to
 * In Puella Magi Madoka Magica, Akemi Homura tried hard to save Kaname Madoka this way. Repeatedly.
 * Bleach: Fullbringers manipulate souls, and according to them all matter has souls.

Comic Books

 * A variation occurs in Superman Batman Generations III:

Fanfiction

 * In Kyon: Big Damn Hero, Kyon has to travel several days back to prepare countermeasures when.
 * At one point the SOS Brigade is in an urgent need of a dimensional anchor. Immediately after Kyon realizes one may be in Tsuruya's possession there's a knock on the door: Tsuruya just arrived to the clubroom to deliver the dimensional anchor, as per requested by Kyon('s future self).
 * Harry Potter and The Methods of Rationality plays Xanatos Speed Chess with this trope.

Films

 * Not only is it used in Bill & Ted's Excellent Adventure, it's practically elevated to a martial art in Bill & Ted's Bogus Journey, where the climax has both the titular heroes and their nemesis making use of this trope.
 * In Paycheck, Ben Affleck's character does this not by going back in time, but by seeing the future before being stricken with amnesia.
 * The Back to The Future films are an example of a universe where this does not work. E.g., in the third film, the only thing they need is a gallon of gas. Marty (from the end of the film) does not go back in time to give a jerrycan to Marty (stuck in 1885).
 * Which makes perfect sense, because he can't. The DeLorean is destroyed immediately upon returning.
 * Ah, but if he'd received a jerrycan of gas, the DeLorean wouldn't have been on the train tracks when it returned to the future (since the train wouldn't have been necessary), thus wouldn't have gotten smushed, and thus would have been available to take the gasoline back.
 * Similarly, the first movie's problem is not solved by personal-future-Marty showing up with more plutonium, but this could have been a result of Marty deciding that his 1985 is better off thanks to his improvising in 1955.
 * The third movie is a sort-of case. The 1985 Doc Brown is zapped from 1955 to 1885, leaving Marty in 1955 without a time machine. And Doc can't fix the machine to go back and get him, so he simply leaves it buried in a mine, and has a letter sent for that particular time and place, informing Marty of the situation and including directions for 1955 Doc to fix it. So from Marty's point of view, as soon as Doc vanishes a time machine and instructions for repairing it were suddenly there all along.
 * Actually the Back to the Future universe counts as an aversion of this trope because any interaction with your past or future selves can create a time paradox that can destroy the universe. Granted, that's a worst-case scenario. The destruction might be limited to our own galaxy.

Literature

 * In Artemis Fowl: The Time Paradox, Artemis and Holly use this once to get out of trouble, but a second attempt falls flat.
 * This is the premise of Philip K. Dick's short story "Paycheck".
 * Jack McDevitt's recent novel Time Travelers Never Die makes extensive use of this principle.

Live Action TV
"Amy: You don't know which lever?
 * Doctor Who:
 * Because of this rule, The Doctor was able to save River Song in Forest of the Dead.
 * This is also the Doctor's only recourse in Blink, not so much helping himself as helping Sally Sparrow defeat the Weeping Angels in 2007 because they sent him back to the 1960s without the TARDIS.
 * Parodied in The Curse of the Fatal Death, where both sides attempt this. Repeatedly.
 * Used in Season Finale "The Big Bang", repeatedly, where we first see the Doctor show up and give random orders, leave, come back a second later, give more, and repeat a few times. Later, we see it from the other side, and learn he's doing this in real time in the future as he figures out what he needed to have already have happened. Thanks to the Timey-Wimey Ball in that universe, he probably can't rely on things he's going to do later, so going back and retroactively doing them the instant before he needs them is safer.
 * Used rather cleverly at the very end, when we find out that two events earlier in the series that didn't make much sense at the time (the TARDIS returning to young Amy waiting on the Doctor, despite us knowing that she didn't see him again till twelve years later; and the Doctor telling Amy to 'remember what I told you when you were seven') turn out to be future versions of the Doctor, setting things up so Amy will remember him.
 * Straight-up depended upon by the Doctor in the Red Nose Day special "Space":

Doctor: No, but I'm about to find out.

He gestures dramatically towards a door from which a future Doctor immediately enters the scene.

Future Doctor: The wibbly lever!"

"Jack O'Neill: So... we don't have to go back in time and get the ZPM because... we already went back in time and got the ZPM?
 * Stargate SG-1 two-part episode "Moebius" may not be a straight example, but it probably felt like that to the characters. In the beginning, they need a Zero-Point Module, decide to go back in time to when there was one on Earth, then hide it in a recently-discovered excavation site. At the end, General O'Neill finds himself watching a video of himself and his team explaining how and why they went back in time, which was recently discovered at an excavation site alongside the Zero-Point Module they needed. The only part O'Neill understands is that he doesn't have to do anything now that someone from another timeline did it for him.

Sam Carter: ... Pretty much.

Jack O'Neill: ... Alright then, let's go fishing."


 * The movie Continuum brings up the question: what if both sides have time travel technology? Ba'al attempts to use this rule, but is defeated when Mitchell travels back even further and stops him in the past. Confused yet?
 * The Stargate Atlantis crew probably wouldn't have survived the first episode if it weren't for the efforts of an alternate Dr. Weir and this trope.
 * Averted in Seven Days, when the Russians manage to get their time travel technology off the ground thanks to Olga. Unfortunately, the technology is controlled by a rogue Russian general who proceeds to kill the Russian president and take power, intending to use the Sphere to prevent any attempts to remove him from power. Parker manages to "backstep" and stop the Russian program before their first jump. Interestingly, Olga previously worked on another Russian time travel program, which did not bear fruit but got her recruited into the Backstep program.
 * Kamen Rider Double has an odd variation on this. The Yesterday Dopant has the power to make someone do whatever they were doing exactly 24 hours ago. When Double shows up to fight it, Yesterday specifically baits him into actions that, when affected by Yesterday's power the next day, will cause him to try and assassinate a public figure.

Tabletop Games

 * In the time-traveling RPG Continuum, this is called "slipshanking," and it's a viable alternative to your future self actually showing up to help you out of a jam (a "Gemini"). However, contrary to the Bill & Ted example, the DM should insist that the player arrange the slipshank in-game, or take a Frag penalty. Too much slipshanking is a sign of poor planning, and a rude imposition on your future self who has other things to do.
 * This is one of the bonus options in Time and Temp after you've gathered enough knowledge to come up with a good plan (represented by writing your die rolls into a grid and creating certain patterns).
 * A card in Chrononauts is called "Memo from Future Self". it effectively works like this, instantly negating the last card another player played. The German Chocolate Cake artifact can also be used as a Memo and the image on the card shows it having a postcard attached. Though, Word of God says that it is not the postcard but the cake itself, and that the cake is just so good that it distracts the other player from doing what they just did

Video Games

 * Time Splitters: Future Perfect had numerous examples of this. One of the earliest examples is also one of the most memorable - you are given a key by your future self that you need to progress, and later pass the key on to your past self, leaving its initial existence unexplained. Fridge Logic also sets in when you consider the fact that one key is being infinitely passed from Cortez to Cortez, meaning it'll probably wear down and break at some point.
 * In Achron this is a very basic tactic. If your base is attacked, you can go back and build defenses in preparation.
 * Subverted in Singularity, where the player receives advice from the future in the form of time-reversed chalk marks on the walls. You'd think that would be a huge help, but they wind up not helping because whoever wrote them has/will-have-gone completely bonkers from excessive time travel and can't explain anything coherently enough for the messages to be helpful at the time you receive them.

Webcomics

 * Irregular Webcomic accomplishes this and ensures a Stable Time Loop at the same time.
 * Thomas Overbeck's Times Like This, a webcomic focused on time travel, makes an art out of this.
 * Homestuck: Stable Time Loops are everywhere in this series, and there are devices that can send anything to any point in time as well as devices that can pull anything from any point in time as long as no paradoxes are created. The best example is the Bunny - although it also helps cause the problems it was created to solve.
 * A more subtle example: The trolls have the ability to contact anyone at any point in their timeline - up until a Time Crash event. That Time Crash event turns out to be important, so they eventually help the human characters create the event by telling them what they do in the future and passing on information from their future selves. This eventually leads the humans to do those things and pass on that information to the trolls.
 * Lord English is a time-traveling demon who can only enter a universe once it has died. Once he enters, he can travel back in time and prepare his own summoning.

Web Original

 * The Olde English sketch "Pizza Delivery" parodies this in a Running Gag whereby all the preparation is for the sake of, well, pizza delivery.

Western Animation

 * This trope sets the plot in motion in the Futurama movie Bender's Big Score.
 * In the Kim Possible movie "A Sitch in Time", used the time monkey idol to change a whole lot of things, including making tons of money by buying a big company before the bubble burst, causing Ron to move to Norway, Taking Over The World by mind probing the entire population, travel back to the future!past to tell her past self to steal the Time Monkey Idol.