Ignorance Is Bliss

Princess Toadstool, I know you're frightened,
But if you knew just why you're here
Your fear would just be heightened.
So let's just say...ignorance is bliss!

—Bowser Koopa in the song "Ignorance is Bliss" by Jellyfish, from the album White Knuckle Scorin'

So, the hero has found the giant factory belonging to the Big Bad. He enters with his True Companions and finds weapons that could kill thousands or maybe even millions, all built by and being built by the locals. While the hero believes that the locals all knew about the Big Bad's plan and were helping him willingly, a few questions from The Heart clears up the matter- the locals simply think the weapons are harmless machinery that will be used for building houses and/or farming.

This trope describes situations where somebody uses someone to make or do something by lying about what exactly it is that they're doing, or assuring them that it's for the greater good. It could be something as simple as stealing plans or moving an item to another location, or something major, like building a Doomsday Device or breaching a boundary- if the Big Bad plays on their ignorance to make them do something; it's this. This can often result in a Heel Face Turn or My God, What Have I Done?. See also Evil Plan, Nice Job Breaking It, Hero, Awful Truth, Redemption Equals Death and Ambiguous Innocence.

Examples of Ignorance Is Bliss include:

Anime and Manga

In Black Butler, Ciel tells Snake that his missing friends' whereabouts were unknown and that staying with Ciel is the fastest way to see them again. The reader knows that they were all killed by Ciel's staff when they attempted to ambush the mansion.

Film

Forrest Gump is the Trope Codifier.
Diamonds Are Forever: Blofeld kidnaps Willard Whyte and uses his industrial empire to build and launch a Kill Sat. Only one other person in Whyte's organization knows what's going on: the rest are kept ignorant.
- Also in Diamonds Are Forever, we have a rocket scientist who helped Blofeld to build the Kill Sat because he thought Blofeld would use it to help achieve world peace. Of course, this wasn't the case.
At the start of Van Helsing, Dracula tells Dr. Frankenstein that he did this to him.
Cypher uses this line in The Matrix to justify his betrayal.

Literature

In American Gods Wednesday uses Shadow to start a war between the gods so they can conquer the new gods, when in reality, Wednesday and Loki just want the power they'll gain from a field of dead gods.
The Belgariad: Errand is brought up to be totally innocent so he can sneak into the Rivan throne room to steal the Orb, without knowing what will happen afterwards.
In the Star Wars Expanded Universe the person responsible for designing the Death Star and Sun Crusher has no idea that they'd be used to destroy inhabited planets and the suns around which they orbited, intending for them to be mining vehicles.
Harry Harrison's The Stainless Steel Rat. Jim diGriz discovers that the government of the planet Cittanuvo is building a Warlord class battleship under the pretense that it's just a transport ship. After investigating, he finds out that two elite criminals tricked the government into building the ship with the intent of stealing it and using it for interstellar piracy.
The Howlers in Animorphs routinely wipe out entire species at their master's command so the protagonists assume they must be consumed with rage and enjoy causing destruction. It turns out they think it's all a game.
Ender's Game, because when he goes to the simulator he's actually commanding real troops.

Live-Action TV

In Glee, Sue gets Brittany to give her the set list for New Directions, but Brittany has no clue about what she's going to do with it.
NCIS: In 6.21, "Toxic", Abby builds a biological weapon without knowing what it is.
In Dollhouse, it turns out the Evil Corporation is building a Doomsday Device by making each of their research departments around the world build a little part of it, and nobody knowing about the larger picture.
House had an episode called "Ignorance Is Bliss" where it turned out that the Patient of the Week (who was a genius) was regularly taking cough medicine so he would be around the same intelligence level as his girlfriend.

Newspaper Comics

Dilbert had Wally explain to him how this works.

Tabletop Games

RuneScape has this in the quests "Priest in Peril" and "Devious Minds".
In World of Warcraft Horde-players visiting Dragonblight are offered a questline to help the Forsaken gather ingredients for a plague that will inevitably wipe out the scourge, which Puttress later uses to kill, not only scourge-soldiers, but also the combined army of humans and orcs battling them.

Video Games

Spyro: Year of the Dragon: Bianca helps the Sorceress abduct the dragon eggs thinking they'll be raised in the Forgotten Worlds- the Sorceress really plans to kill the dragons and take their wings for a spell.
In Metal Gear Solid, Otacon designed Metal Gear REX under the assumption that it would be used as a mobile TMD to destroy incoming missiles - he had no idea that his employers would arm it with nukes instead.
In Final Fantasy VI, after the Espers from the sealed cave annihilate Vector, Emperor Gestahl held a banquet in order to make peace with the returners. There, he asked Terra to act as an ambassador to the Espers in order to make peace with them too. Once Terra & co. led the espers out of their hiding place, the recently imprisoned Kefka comes out & turns all the Espers into magicite. All the acquired magicite give Kefka & Gestahl godlike powers, resulting in the rise of the Floating Continent.
This trope is probably the one and only Pet the Dog moment from Matt Engarde in Ace Attorney:

Engarde: (Sigh)... I thought if you didn't know, maybe you wouldn't feel so bad for doing your job...

In Alice: Madness Returns, the loading screen may give this piece of advice: "Ignorance is bliss, except when it's fatal."

Web Comics

Fighter of 8-Bit Theater is an example of this. The other Light Warriors often get him to do things by convincing him it is for the greater good. There's also the fact that he still believes they're heroes after all the untold damage they've caused.
The Inexplicable Adventures of Bob: In an uncharacteristically manipulative moment, in her second appearance, Voluptua lets Bob fly off on what he thinks is a very simple mission, while she suspects an alien supercriminal just might be waiting for him.
In Sinfest Slicky explored this. First he tasted an apple from the Tree of Knowledge, and couldn't deal with this. Then he washed it off with Lethe water and forgot everything. Then he thought that if "ignorance is bliss" and "knowledge is power", he can have both by mixing them into "Forbidden Fruit & Lethe Water Power Drink". This wasn't a good idea.
Vexxarr had repair drones request that the protagonist "tell the truth rather than try to win our loyalty with platitudes and assurances". It took him one phrase to make them hastily reconsider.