When there is a group of characters who are all just totally weird, either in general or in a particular scenario, the Only Sane Man is the only one who, well, isn't.
Picture this: Alice is the Stupid Evil Psycho for Hire, Bob is a Cloudcuckoolander, Henry is an Empty Shell, Charlotte is a Chaotic Stupid prankster, Karl is too nice for his own good, Daniel is Charlotte's Annoying Younger Sibling, Emily is a shallow sociopath, Maria often Rhymes on a Dime, Franklin is a Mad Scientist, Gardenia is a Lawful Stupid Fundamentalist, Jacob thinks he's a hero on a grand quest, and Lily is a Mad Artist who's obsessed with her wax "statues." Looks like your standard Dysfunction Junction. But then you have Isaac. Isaac is actually a very well-adjusted individual. He reacts with appropriate horror to things like Alice's finger and eyeball collection, Gardenia's tendency to attack anyone not believing in her religion, Franklin's experiments to revive the dead with science, and the crimes against nature that Franklin calls pets. Or reacts with either annoyance or bewilderment to things like Bob's warped logic, Karl's Honor Before Reason mindset (honor being always thought first in his case), Maria's insistence to rhyme all the time, and Jacob's delusions of being a hero. Isaac is the Only Sane Man and The Only Voice Of Reason in the room.
The other variant is where the other characters aren't always that weird, but everyone save one character is acting weird in a particular situation. For example, they might regard something absurd as Serious Business, with the Only Sane Man the only one who notices how crazy that is.
This latter variant also includes a standard comedy piece: something absolutely insane is going on, but only one person notices (or cares). There are usually three stages, with a rough correspondence to the Five Stages of Grief: Bewilderment (Shock and Anger), trying to get others to see or admit the weirdness (Bargaining and Denial), and bitter sarcasm (Acceptance).
In the more extreme cases, the poor soul may be trapped in a World Gone Mad and/or wind up Giving Up on Logic in frustration. Sometimes, though, they are Not so Above It All. A character may also think of themselves as the Only Sane Man without proper justification.
A Too Dumb to Fool character may be the Only Sane Man, although he is likely to be less worked up about the failures of others to see than in most cases.
The Only Sane Man is often relied upon as the Only Sane Employee. This character often ends up a Knight in Sour Armor. For a more horrific version, see Through the Eyes of Madness. For the sci-fi version, the Ignored Expert is your go-to guy. The comedy version will often end up being either a Butt-Monkey or a Chew Toy. If all the other characters are otherwise sane, and really should see something, but only one guy does, he's an Einstein Sue. See also Cassandra Truth, Surrounded by Idiots, and Surrounded by Smart People; contrast with the Unfazed Everyman. May temporarily overlap with What the Hell, Hero?.
If several characters take turns being the Only Sane Man, they're playing with a Sanity Ball. If it's a two-person show, with one person playing the Only Sane Man to the other's wackiness, it's a Straight Man and Wise Guy scenario. This trope is the savvy half of a Savvy Guy, Energetic Girl duo, one third of a Comic Trio, and part of the Four-Man Band. If they're only later proven right, it may be The Dissenter Is Always Right.
Usually, but not always, a Cast Full of Crazy contains an Only Sane Man. Usually, he's prone to Sanity Slippage. Sometimes, he's Only Sane by Comparison, and in more "normal" casts he'd be considered the craziest of them all.
The Wonka may well feel like this (or be this!) in a world that doesn't follow his thinking. Insanity is, after all, in the eye of the beholder, and The Wonka often has quite valid points.
No Real Life Examples, Please! - Real Life does not have only one sane person.
