More common in works revolving around warfare, a Mary Tzu is a character, usually a military commander, with unrealistic tactical abilities. She (or he; actually more often he, [[JustForPun we just couldn't pass up the pun]]) can pull a win out of any battle no matter how outnumbered, outgunned, or outmatched, she can guess any enemy commander's plan no matter how convoluted or how little information she has to work from, and any plan she comes up with will work perfectly, often employing HollywoodTactics, resulting in total enemy defeat and another smashing success for General Tzu's track record. As per the standard MarySue traits, such a character is usually treated as a paragon and/or revered by the other characters. Sometimes she comes complete with a BigBookOfWar, possibly written by her.
To be clear, this trope only deals with ''unrealistically'' good tacticians. ''Merely great'' commanders wouldn't apply, as long as they're fallible, or if they're written well enough to make their victories seem plausible. If it looks like they play GambitRoulette instead of [[ThePlan more sound and realistic plans]] you've got a Mary Tzu. Another test you can do: Are the character's victories all perfect? Or does he suffer losses and pay prices? Even if there isn't a GambitRoulette every other victory, having plans that are unrealistically successful, such as losing ''no'' troops who were sent in to distract heavy fire, that's also a Mary Tzu.
In some respects, one could argue that this is a case where [[ShowDontTell "show, don't tell"]] does '''NOT''' apply. If a character is simply described as an excellent commander, but the writer doesn't actually describe the tactics involved, it's a lot easier to accept, since the actual maneuvers can be left to TakeOurWordForIt.
Sometimes, this character will merely be a plot device, but other times it can be piled onto the CommonMarySueTraits of your [[ImprobableAge fourteen year old]] [[EverythingsBetterWithSparkles sparkly]] [[GodModeSue invincible teenage girl]]. Those are only worst case scenarios though, as most characters associated with this trope tend to be older men (which sometimes makes them popular despite being [[BoringInvincibleHero invincible]]).
In Real Life cases exist of brilliant strategists successfully predicting their opponent's plans, only to then dismiss their own prediction as depending on ascribing Mary Tzu-like powers to their enemy.
Named for SunTzu, a RealLife (maybe) [[http://en.wikipedia.org/wiki/Sun_Tzu Ancient Chinese general]] whose ''[[TheArtOfWar Art of War]]'' is still used by many militaries around the world and is considered one of the greatest treatises on warfare ever written, as well as MarySue, the archetype of the unrealistically perfect protagonist.
Compare to TheChessmaster or TheStrategist, which are for masterminds that push, but don't destroy, WillingSuspensionOfDisbelief. Also see HollywoodTactics and IdiotBall as (like all MarySue types), Mary Tzu's main ability is to make everyone else incompetent so that she looks good.
'''No examples please, this just explains the term.'''