A Time Travel trope, Retroactive Preparation is an intentional use of You Already Changed the Past 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 to set that up, 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, if only you'd thought to leave a Key Under the Doormat or something!
But wait! You have the next best thing: A time machine!
Secure in that knowledge, you look under the doormat and discover the key — it's been left there for you by your future self. Thanks to future-you, you've been prepared the entire time.
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. After you "discover" your key and watch your show, you just make sure to use the time machine and put the key under the doormat for your past self.
Of course, there's no need to actually show anyone setting it up, just the end results.
If it is shown getting set up, sometimes a very strange thing will happen: The time traveler might end up rigging both the way past and original obstacle, e.g., when you're putting the key in place, you discover the door was left unlocked, and you know it 'was' locked, so you lock it.
- In The Disappearance of Haruhi Suzumiya, Kyon mobilizes the SOS-dan to come back in time and save himself from Asakura and fix the whole parallel universe thing.
- A variation occurs in Superman and Batman Generations III: Darkseid's plan to conquer Earth starts by sending an invasion fleet to attack the planet. Should they fail, the survivors time travel back 100 years and try again, and so on and so forth.
- Loki in Loki: Agent of Asgard went back in time (or maybe in story) to create the sword that was used to free Thor from their parasitic influence, because they wanted to wreak havoc without the restrictions of, well, basically Demonic Possession. In this case the time loop wasn't as stable as expected though so it invited the Butterfly of Doom over unintentionally creating Verity Willis.
- In Child of the Storm and its sequels, Doctor Strange is the grand master of this trope, exploiting it to manipulate everyone. By the sequel, it's got to the point that Thor just assumes (accurately) that Strange has a plan, one that he'll have been spending centuries refining, "Because he is Doctor Strange and that is what Doctor Strange does". He therefore proposes to solve an apparently unsolvable problem by the simple expedient of picking Strange up by the ankles and shaking him until the answers come out. It actually works, in epic fashion.
- In Kyon: Big Damn Hero, Kyon has to travel several days back to prepare countermeasures when Sasaki is kidnapped.
- 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).
- Not only is it used in Bill and Ted's Excellent Adventure, it's practically elevated to a martial art in Bill and Ted's Bogus Journey, where the climax has both the titular heroes and their nemesis making use of this trope before pointing out that only the winner of the showdown can actually make use of it.
- In Tenet, this happens during the Final Battle. The Protagonist and Ives arrive at the cave, but there's a locked door between them and Sator's right-hand man Volkov, who is burying the Algorithm for the people from the future to find. Fortunately, an inverted corpse springs back to life, takes a bullet meant for The Protagonist, and opens the door, allowing The Protagonist and Ives to overpower Volkov and retrieve the Algorithm. When the battle is over, it's implied Neil will eventually invert so he can open the door and take the bullet.
- In Artemis Fowl: The Time Paradox, Artemis and Holly use this once to get out of trouble, but a second attempt falls flat (Artemis muses that It Only Works Once). After the main plot has been resolved, a few sentences are devoted to Artemis getting no.1 to leave the help for their past selves.
- This is the premise of Philip K. Dick's short story "Paycheck".
- Jack McDevitt's novel Time Travelers Never Die makes extensive use of this principle.
- So does The Man Who Folded Himself by David Gerrold, the time traveler using it even to get "immortal".
- Subverted in The Riftwar Cycle where Pug sometimes receive instructions apparently written by himself in the future. They are actually written by the god Kalkin.
- We will never know for sure, but this appears to be what's going on in the surviving fragment of The Salmon of Doubt, with a number of things making a lot more sense if you assume that a Dirk Gently from the future is making sure present Dirk ends up in the right place.
- Subverted in the Doctor Who New Adventures. The Doctor walks up to a fridge, and proclaims "This fridge will be full of delicious food: in the future I will travel back in time and put it there." He opens the fridge; it's almost empty. "I forgot," he adds. Another book has the Doctor stymied because the Big Bad, knowing his methods, has removed all the notes he was expecting his future self to leave for him.
- In The Overlords Of War by Gerard Klein, the main characters need to depose an alien dynasty distinguished by laying blue eggs. They travel to the past when the current ruler had just hatched, replace the shards of his egg with another, but painted, and then, in the present, expose it as a fake, making him appear illegitimate.
- Doctor Who:
- The Seventh Doctor became particularly associated with this, mostly due to "Battlefield", in which he finds himself resolving an issue created by a future incarnation, and "The Curse of Fenric", in which he's been fighting Fenric on multiple levels in different time periods. By the time of the Doctor Who New Adventures, it had become memetic that he always did this, so the books started playing with it (see above).
- Parodied in the non-canonical The Curse of Fatal Death, where both the Doctor and the Master attempt this, repeatedly going back in time to bribe the architect of the castle they're in to plant a series of traps to be used against the other.
- This is 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 1969 without the TARDIS.
- Because of this rule, the Doctor was able to save River Song in "Forest of the Dead", as he realised that, knowing about her death, he now had plenty of time to find a way to save her.
- Used in the Series 5 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.
- Rather cleverly at the very end, 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. Well, actually, it's the Doctor reversing through his own timeline as he gets erased from time, so he's really just taking advantage of involuntary time travel rather than having planned it, but it works the same way.
- Straight-up depended upon by the Doctor in the Red Nose Day special "Time":
- Taken to ridiculous levels in "The Day of the Doctor", where the Doctor pulls off a thousand-year preparation in order to save Gallifrey by placing it in a time lock.
- Taken advantage of by Missy in "The Doctor Falls". When her previous incarnation reveals that his/their TARDIS is inoperable due to a broken piece of Phlebotinum, she invokes this trope. She throws said previous incarnation against the wall, and claims that she "remembers" a very scary lady once throwing her against the wall and forced her/him to promise to always carry a spare piece of that phlebotinum at all times. The promise made, she then reaches into her own pocket to reveal that she now has the exact piece of phlebotinum they need.
- "Spyfall": The Thirteenth Doctor uses this to save her companions' lives when they're left on a crashing, cockpit-less plane by the Master in the first half's cliffhanger. After being reminded to do so near the end of the story, the Doctor travels back in time to the construction of the plane to plant notes and features so Ryan knows to plug his phone into the plane's computers to control it.
- 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.
- 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.
- 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.
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?
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 to stop the Stargate Program being formed in the first place by travelling back to 1939 and sinking the ship bringing the Stargate to America, but is defeated when Mitchell travels back even further to 1929 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.
- 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"). Contrary to the Bill & Ted example, an immediate frag penalty is assessed which can only be resolved by somebody (usually yourself) travelling back in time, obtaining the object elsewhere, and placing it for you to find. 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 Your 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. (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. Presumably the postcard is just to tell yourself when to use it.)
- TimeWatch: All characters have the Preparedness ability. Technically this can be used for such mundane things like "of course I remembered to pack spare batteries", but it's far more likely to be used for "tomorrow, I'm going to go back five months, make a copy of the bunker key, and hide it inside the third ventilator". Preparedness does not prevent Paradox issues, though — if the third ventilator had already been searched, finding the key inside it with a Preparedness test means the PCs get to roll a Paradox test as well.
- TimeSplitters 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.
- The Kingdom Hearts series is the result of the Big Bad Xehanort doing this to himself. He uses his past self in his schemes to get the X-Blade, thus planting a seed of darkness in his past self's heart, which acts as a catalyst to eventually corrupt him into the monster that he becomes, who eventually uses his past self in his schemes...
- Nazi Zombies has a doozy, which doubles as a Stable Time Loop. The crew of Dempsey, Takeo, Nikolai and Richtofen save a girl stuck in an alternate dimension using ancient replicas of magical staffs in 1917. Over the course of several years they then endeavor to save their own souls as child-versions for a world free of evil. This pure world is then infested with evil, which the heroes then defeat; only for the deity of the world to attempt to erase them from existance, before dumping them in a 'corner of time alongside some other random stuff'. Said point in time ends up being the war where they build magical staffs to fend off the evil, thus creating the very moment that they were drawn to in the first place. Since the staffs were explicitly replicas modelled after the originals, they knew they needed to make the originals themselves.
- In Neverwinter Nights, you need to fight some nearly invincible golems at one point. You can travel to the past and convince the slaves working on them to introduce vulnerability to a certain type of damage.
- 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.
- Dave, the Knight of Time, has this as his modus operandi, using time duplicates to aid him in everything from combat to stock trading scams. As time travel in Homestuck is semi-deterministic (you CAN change the timeline and create a paradox, but doing so causes you to branch off into a doomed timeline where everyone dies), Dave has to be very careful that he does everything that he's "supposed to do" after he's seen his future self do it. The question of where his future selves' actions originate in the first place is brought up, and he admits that he operates mainly on instinct and tries not to think about it too hard. Eventually he decides the pressure of keeping all his loops stable is too much and decides to lay off time travel for good.
- 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. Makes sense, since he's a fellow Time player like Dave.
- In the Kim Possible movie "A Sitch in Time", Shego 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, and travelling back to the future!past to tell her past self to steal the Time Monkey Idol.
- The Rick and Morty episode "Rattlestar Ricklactica" has Rick be obviously smart enough to use this method when he is dragged into Time Travel shenanigans, but it turns into an effective but needlessly unpleasant execution of the trope due to his abrasive self-sabotaging personality and irritation with Morty. After Morty inadvertently alters the course of evolution for a planet of sapient snakes, whose warring future factions send a Terminator Twosome after the family, Rick's plan to put a stop to the whole thing on Snake World encounters a snag as he's confronted with a problem that will take several hours to solve, so he makes a mental note to "commit even more to [Morty's] f**kup". Seconds later, a second Rick and Morty walk into the room carrying the supplies needed for the next stage of Rick's plan. The future Rick and Morty are extremely rude to their counterparts as they hand things over, and future Morty sports a black eye. It turns out that while their preparation helped them then and there, they wind up having to put off celebrating Christmas later on to put things together, basically for no reason except to fulfill the Stable Time Loop where Rick winds up being the cause of Morty's black eye a few hours later.
- Used twice in Gargoyles (which runs on strict Stable Time Loop logic) by characters to give themselves extreme wealth/power.
- Xanatos went back in the past, contacting a long lasting secret society, giving them some coins from this time as well as a letter, with explicit instructions to give himself the coins/letter in the future. The coins (as well as instructions how to best use the wealth from it) form the basis of Xanatos' massive wealth. The letter also includes instructions on how to travel back in time to give himself the new letter. Also it is implied that part of this scheme exists just so Xanatos can tell his father that he is technically a "self made man."
- A wizard uses the same time travel device to basically turn himself into a God. He uses time travel to save himself from death, gives his past self allies, massive amounts of magical power, teaches himself everything he needs to know, then sends the newly empowered self back in time to redo the process.
- The Catholic Church eventually adopted the doctrine that this happens/has happened/will happen when they pray for the souls of the dead. The official line is that God hears their request in the present and, since it's eternalnote , actively manipulates the person prayed for a little bit so they'll be just that bit closer to repenting and thus not spending as long in purgatory before going to heaven/not ending up in hell and staying there forever.