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 a Psycho for Hire, Bob is a Cloudcuckoolander, Doug is an Empty Shell, and Emily is a Mad Scientist. Looks like your standard Dysfunction Junction. But then you have Gardenia. Gardenia is actually a very well-adjusted individual. She reacts with appropriate horror to things like Alice's finger collection and the crimes against nature that Emily calls pets. Gardenia is the Only Sane Woman.
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.
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- Exit Tunes Presents ACTORS: Hinata Mitsutsuka. Despite his character design and cheerful personality, he is in fact the one who reins in his fellow club members' more...unorthodox ideas.
[talking to the Singing Club, about the cat the Singing Club had accidentally adopted] "Like I said, that kind of naming sense is no good!!"
- In ACTORS2, Rei Ichijoudani. While he has his moments where he acts otherwise, for the most part, he acts like this. Especially when it comes to his former nanpa partner Takato...
- In ACTORS3, Seijun Yuyama, mostly with regards to his Club President Kouya and friend Kaoru.
Seijun: Don't drag me into this conversation, you perverts.
Kaoru: I'm no pervert! I'm a Covert Pervert!
Seijun: Don't just say that so easily!!
- Red vs. Blue:
- Played straight with Lopez the Heavy. After a few years of dealing with the Armed Farces all around him, he more or less resigns himself to dealing with it with a hefty dose of sarcasm, even for this World of Snark.
- It also repeatedly has this as a Subverted Trope. If someone is put in place as the Only Sane Man he often becomes The Leader by default... but is also one of the only members of the cast with a legitimate mental disorder.
- Alpha!Church is revealed through a series of Cerebus Retcon situations to have a severe case of a Literal Split Personality brought on by extensive Mind Rape.
- Washington more or less lost his mind (for a time) when Epsilon was seemingly Driven to Suicide while still linked with him. When he gets better through extensive Character Development, he turns out to have quirks just like the rest of the cast.
- Felix seems to be rather okay as a new guy in Red vs. Blue: The Chorus Trilogy... but it turns out that he seems to have a checklist of traits of antisocial personality disorder.
- Supermarioglitchy4s Super Mario 64 Bloopers: Starman 3. While he does have his moments, he has the least amount of plans that are unsafe.