It's a Long Story

"Let me explain. [Beat] No, there is too much. Let me sum up."

—Inigo Montoya, The Princess Bride

This is a phrase typically used when a character has finished an adventure isolated from the others. Since it is redundant to have him/her explain the events we just saw ourselves, the typical move to have the character answer the others' questions of what happened is with, "It's a long story" with the assumption that he tells it to them later, off-screen.

A variation can be used when the writers need to create a sense of mystery or suspicion about the recent off-screen past of a character. He will reappear or be found/rescued and when questioned by his compatriots will use this line and refuse to elaborate until the court-martial. "It's complicated" works the same way when it's used like this.

Occasionally subverted / parodied when one character says this in response to being questioned as to what is going on, only for another character involved to sum everything up in one sentence, thus proving that it's not actually that long at all.

Another subversion is for the person on the receiving end of the story to say "I have time."

This phrase can also be used as a substitute for the explanation of a Noodle Incident.

Compare It Makes Sense in Context where this trope is used but not invoked.

Examples of It's a Long Story include:

Anime and Manga

Kanon Ayu tells Yuuichi several times that the reason she's stole Taiyaki is a really long story. He keeps pestering her about it until she gives up and tells him I was hungry.

Comic Books

Subverted in The Adventures of Tintin: in Land of Black Gold, Haddock meets Tintin at the end, and is very adamant in his repeated attempts to explain what happened to him offscreen ("It's quite simple - and, at the same time, rather complicated..."), only to be interrupted every time. Eventually, he gives up, Breaking the Fourth Wall to tell the reader "you'll never know!"
In the introduction of Empowered, why the lily-white Ninjette has a Japanese name.

Fan Works

In Kyon: Big Damn Hero after Kyon returns to the clubroom with a little cut in the head, a faint line of blood trickling down one cheek:

Haruhi: What happened?
Tsuruya: You're still bleeding? But I put a bandage on that for you yesterday!
Kyon: Long story.

In Luminosity, Bella (now a vampire) is called in to help the werewolf pack she activated earlier. When she gets there, she finds the pack in an uproar.

"So..." I said. "What's all this, then?"
"It's a very, very long story," Rachel said stiffly.
"I'm immortal," I said.

Film

Spoofed in Dirty Work:

Kathy: Wait, you're brothers?
Mitch: Yeah, it's a long story.
Sam: My dad boned his mom.
Mitch: Okay, it's a short story.

Near the beginning of the German movie Im Juli, Daniel attempts to use this as an excuse not to tell the guy he's hitchhiking what happened. The guy then points out they're going to be in a very long car trip anyway, so he might as well start. Then we go to the actual plot of the movie.
Played uncomfortably straight in the appropriately-titled Paulie as he is talking to the janitor.

Paulie: I'm warning you, its a long story.
Janitor: I'm Russian. I like long stories.
Paulie: Okay, Chekov, sit down.

From The Wicker Man, "Do sit down, Sergeant. Shocks are so much better absorbed with the knees bent."
Awesomely spoofed in The Muppet Movie when Fozzie starts to reiterate the whole plot so far to the Electric Mayhem. Kermit decides to save time by letting them read the script.
In Star Trek: Generations, Geordi and Data go to Ten Forward after the former installs the emotion chip in the latter. Data tries Guinan's latest syntheholic beverage and makes a face. She asks him what he thinks of it, and he says he's not sure because he doesn't recognize the emotion.

Guinan: Emotions?
Geordi: I'll explain later.

In Uncle Buck, Miles grills Buck with a Long List of questions, including:

Miles: Are you married?
Buck: No.
Miles: Why not?
Buck: It's a long story.
Miles: Do you have any kids?
Buck: No.
Miles: Why not?
Buck: It's an even longer story.

Literature

Goodgulf in Bored of the Rings: "Well, once out of the pit..." (chapter ends)
- Later, when Eorache asks how he escaped: "It is a long tale," said Goodgulf, taking a deep breath. "Then save it," interrupted Eorache."

Live-Action TV

Spoofed in (what else?) Buffy the Vampire Slayer:

Willow: Oz is a werewolf.
Buffy: It's a long story.
Oz: Got bit.
Buffy: Apparently not that long.

And later in the same season:

Xander: Oh God, what'd you do to each other?
Buffy: Long story.
Cordelia: Got hunted.
Buffy: Apparently not that long.

And then next season on Angel. (Yes, all three episodes are by the same writer too.)

Angel: It's complicated how this all happened, Buffy, you know? It's kind of a long story.
Buffy: Your new sidekick had a vision, I was in it, you came to Sunnydale?
Angel: Okay, maybe not that long.

Functional equivalent: In Star Trek: Deep Space Nine, Dr. Julian Bashir awakens from a coma (in which he has had a vivid dream about his crewmates and battling through the station to save his own life) with the words "You will never believe where I've been."
- DS9 uses it several times, but only rarely plays it straight. Worf in particular uses it more than once—because he doesn't like talking.
In The Fresh Prince of Bel-Air, Will's "family" spend the evening with an associate who indulges in unending tedious anecdotes, boring them all. So when Will explains that his recent hijinks are "a long story" they just decide not to probe him on it.
Farscape:

Chiana: What happened to you guys? Why... why didn't you call in?
D'Argo: Well, the restaurant, it, uh, sort of burned down - I don't want to talk about it, it's a long story.

Played with somewhat in Eureka where Sheriff Carter is stuck in a time-loop, but every time the loop resets the energy effect injures him. At first it's minor, but after one of the later resets he comes out of his bathroom covered in cuts and scratches and tells his daughter it's a long story and he'll explain later.
In an episode of Grey's Anatomy, Arizona and Mark hem and haw about why they're with Callie on her ob/gyn visit, until:

Callie: *points to Arizona* Lesbian lover. *points to Mark* Baby daddy.
Dr. Fields: Aaand everyone's a little wigged out.

On the Heroes series finale, Peter rescues Sylar from a nightmare prison inside Sylar's head. When Parkman, who had put Sylar in the prison, demanded how they escaped, the two of them curtly reply that it's a long story.
The scene in the Firefly episode "Heart of Gold" where Nandi kisses Mal, doubling as a Call Back to "Our Mrs. Reynolds":

(short kiss)
Nandi: What is it?
Mal: Just waitin' to see if I pass out. Long story.

Subverted on The Muppet Show after Captain Link is injured on the set of a "Pigs in Space" skit trying to plug a hole in the ship with his nose:

Den Mother: What happened to him?
Kermit: It's a long story... Actually, it's a short story. He did something dumb.

This was the entire plot of How I Met Your Mother. The whole thing is a father telling his kids about how he met their mother.

Radio

The Hitch-Hiker's Guide to the Galaxy, Fit the Eleventh:

Hurtenflirst: Why do those other two chicks we picked up look exactly like you?
Lintilla: It's a long story.
Hurtenflirst: Quick précis then.
Lintilla: Because.
Hurtenflirst: That's neat!

Toys

Bionicle:

Axonn:Just so you know... Brutaka's his own "we" now. Long story."

Video Games

Conker says this in at the intro to Conkers Bad Fur Day, when he begins to explain to the viewer how he became a king and who all the strange people surrounding his throne are. The rest of the game takes place during the previous day.
In Kingdom Hearts II, when the gang are in Christmastown, ~~Sandy Claws~~ Santa Claus asks Jack "What trouble did you bring this time?" Sora asks Jack about it, and Jack replies, "It's a long story!"

Web Comics

Served with a bit of Lampshade Hanging in this strip from Freefall.
In Schlock Mercenary, after the battle with diamond beetles on the Princess Tyola:

Dr Bunnigus: Why raid food stores? Why not use our original bodies for mass?
Schlock: You don't want to hear it. It's a long story.
Dr Bunnigus: Try me.
Schlock: Well, after I ripped everybody's heads off and froze them, I ate your bodies so I'd be big enough to take on the beetle-monsters.
Dr Bunnigus: ... Where does this story start being long?
Schlock: Did I say long? I meant disturbing.

Charby the Vampirate sums up Mye's terrifying night with chibi drawings. It is long indeed, as it started a good 75 pages earlier.
It's Walky! does the big setup version here:

Rachel: Okay, Joe, I think it's time we told you teh sad truth Robin has had to face throughout her childhood. It's a long and sordid tale, and every twist and turn will break your heart.
Agatha: Robin's father slept around.

Web Original

This is Captain Susano's response in Alternate History: The Series upon being asked why his Cyborg Robots With Green Blood have started acting like The Three Stooges.
This is how Chester A. Bum handwaves some of his more... esoteric claims.

I had no genitalia once! Long story.

This is the final line of the "Yellow" Trailer for RWBY, uttered by Yang Xiao Long. It is also a pun on her name, as the trailer is a story about her. It eventually becomes a repeated line through the series.

Western Animation

In the Gargoyles episode "The Price", Hudson escapes from Xanatos' captivity and finds that clan is mourning the destruction of a statue of himself they thought was him in a permanent stone state. When he reveals himself, the others naturally ask what happened and Hudson responds, "It's a long story and one best told over a cup of hot tea" after which they go back to the Clocktower for that tea.
Justice League: In "Hereafter", Superman is blasted into the future. The world mourns his loss and the league has to stop supervillain rioting in Metropolis. Meanwhile Superman has a nature hike across a crazy post-apocalyptic landscape, and fights giant cockroaches with Heel Face Turn Vandal Savage as they try to power up a time machine. Superman appears a few days later with a beard, ragged clothes and pops up just in time to take a bullet meant for Batman. What happened to him? "It's a long story."
- Even better, he starts to try to explain, but it's all: "and then Vandal Savage and I fought some giant cockroaches" and he gives up because everyone's just staring at him. And his...wolfskins and his beard. He left the sword behind, though.
In one Winnie the Pooh animated episode, Pooh's friends think he's moving away, so they hold a going-away party for him and see him off. Pooh never intended to move away, but plays along and returns back home after about two seconds. His friends ask him why he's come back so soon. Pooh replies, "It's a long story," probably because he can't think of a good explanation (or for that matter, fully understand it himself as he's a bear of little brain) of how they were mistaken.
In The Powerpuff Girls episode "The Bare Facts", the Mayor thanks the girls for saving him from Mojo Jojo but asks why they were laughing at the time. They say it is a long story, while the Mayor has time to listen. The three explain the events in Rashomon Style but leave just before revealing what was so funny. While it is shown that it is because the Mayor has no clothes on, that means the girls must have returned him to his office naked.
In the episode "War Dawn" from The Transformers the Aerialbots go back in time and meet Optimus Prime nine million years in the past. When they return, Optimus remembers the incident. Ratchet asks what they're talking about, and Prime says, "Well, it all started about nine million years ago," prompting Ratchet to quip, "Oh great, it's gonna be one of those long stories..."
In one Danger Mouse episode, Penfold basically turns his back for a moment and then sees that D.M. has escaped from the trap he was just in, and fixed their car as well. Penfold asks how he did it, and D.M. says, "I'll explain... later, Penfold." Penfold: "But how did you fix the car?" D.M.: "I'll explain later, Penfold!"
This is Yang's last line in the "Yellow" Trailer for RWBY.
In the Five Episode Pilot of Teenage Mutant Ninja Turtles this is Krang's explanation to two of his generals (from Dimension X) as to why he is now a disembodied brain.