''The Bat'' is a mystery play by Mary Roberts Rinehart and Avery Hopwood, based on Rinehart's earlier novel ''The Circular Staircase''.

The place is the country house of Courtleigh Fleming, the late President of the Union Bank. When his nephew Dick rented it to Cornelia Van Gorder, a spinster of 60, she expected her stay there to be a restful four months, enlivened by the occasional guest and some homemade elderberry wine. Instead, intruders and anonymous threatening letters left her hardly able to sleep at night, and newspaper stories about a mysterious nocturnal criminal known as "The Bat" only added to her worries. These disturbances were already too much for the weak nerves of Lizzie Allen, maid to Miss Van Gorder for twenty years, who would have packed up and left if Cornelia had allowed her. The cook, parlormaid, and housemaid had nothing stopping them from being frightened away from the house; each offered an unbelievable excuse for leaving. Besides Lizzie, the only servants left in the house were Billy, Courtleigh Fleming's Japanese butler, and Brooks, allegedly a new gardener but obviously no gardener at all.

Cornelia phones Police Headquarters, and a detective, Mr. Anderson, is sent over to the house. Arriving before the detective are Dale Ogden, Cornelia's pretty young niece paying an unanticipated visit, and Dr. Wells, the doctor who recently witnessed Courtleigh Fleming's death in Colorado and seems all too eager for Cornelia to leave the house with him. Cornelia, worried but determined to stay, orders the butler to let no more visitors in after Mr. Anderson, but more break-ins will happen later.

One unanticipated visitor is already in the house: Jack Bailey, alias Brooks, the cashier accused of the million-dollar theft which caused the Union Bank's recent failure. Secretly engaged to Dale, he tells her that it was Courtleigh Fleming, not him, who took the money out of the bank and stashed it in a Hidden Room built into the house. Dale gets the idea that Fleming's nephew would know where the blueprints are, and places a telephone call. Richard Fleming is driven over to the house in a hurry by a lawyer named Reginald Beresford. Dick finds the wanted blueprint, struggles over it with Dale, and is shot and killed. The shot came from the same staircase which Lizzie had earlier seen a ghost going up.

As the detective starts asking questions, a strange voice is heard groaning dreadfully over the house phone. The CallingCard of The Bat appears in the house, reminding Lizzie of Anderson's words in the paper: "He may be a merchant--a lawyer--a doctor, honored in his community by day--and at night a bloodthirsty assassin." The doctor makes a suspicious grab for the scrap of blueprint found in Richard Fleming's hand. Cornelia decides to do some detective work of her own, concerning the staircase. And an unknown man staggers in, not knowing who beat him and tied him up or even his own name.

The play was first produced in 1920, and became an international hit. It spawned a {{Novelization}}, and was adapted to film in 1926, 1930 and 1959. The 1930 film adaptation, titled ''The Bat Whispers'', was cited by Bob Kane as one of the inspirations that led him to create ''Franchise/{{Batman}}''.

Not to be confused with ''Theatre/DieFledermaus''.
* AffablyEvil: Dr. Wells in the 1959 version. He murders John Fleming when Fleming admits he embezzled one million from his bank, followed by suggestion that Wells should help him fake his death so he could get the money in exchange for half, or he'd kill Wells to keep him quiet. Beyond that, any evilness on his end appears as him warning Cornelia to get out of the Oaks because of its bad history so he can get the money, [[spoiler: and it turns out he's not the Bat.]]
%%* AllAsiansKnowMartialArts: Billy
* BigDamnHeroes: [[spoiler: Warner, in the 1959 version, arrives and shoots The Bat, revealed to be Lieutenant Anderson before Anderson can murder Cornelia, Lizzie and Dale.]]
* BitchInSheepsClothing:
** Dr. Wells murders John Fleming to steal the million Fleming embezzled, though Fleming planned on murdering Wells if he didn't cooperate.
** John Fleming embezzled a million dollars from his bank and framed the cashier and planned on murdering Wells if he didn't cooperate (and may have been planning to murder him anyway to silence a possible witness).
** [[spoiler: Lieutenant Anderson is The Bat]].
* CallingCard: The bat symbol comes in many forms.
* ChangedMyMindKid: [[spoiler: Warner in the 1959 version arrives just as The Bat is about to LeaveNoWitnesses. Warner then explains he was on his way to the airport and changed his mind.]]
%%* CoolOldLady: Cornelia, especially in the 1959 version.
* FauxAffablyEvil: John Fleming from the 1959 version. He starts off a rather amicable conversation with Wells about how he stole the money and framed the bank manager, but then leads into a cold blooded discussion about murdering an innocent man and disfiguring the body so John can fake his death and retrieve the stolen money. If it hadn't been for that forest fire catching him off guard, John may have very well killed Wells if he didn't cooperate (and possibly even if he did).
* GenreSavvy: Cornelia applies her expertise in mystery novels to what's going on around her concerning the Bat and the missing money.
%%* HandyCuffs: Performed twice on the same person.
* HollywoodGlassCutter: In the 1959 version, the Bat uses a small blade to cut a hole from a glass-paned door, which he then unlocks and opens by reaching inside.
* HystericalWoman: Lizzie. While this is still there in the 1959 version it's also somewhat downplayed. One reviewer has claimed the 1959 Lizzie was much more bearable than the previous two.
* MamaBear: In the 1959 version Cornelia begins to take on this role for Dale after [[spoiler: Judy is killed.]]
* MysteryWriterDetective: In the 1959 film version, Cornelia is rewritten as a mystery writer renting the mansion for some inspiration, which she gets in spades.
%%* OldDarkHouse
%%* OuijaBoard
* PlatonicLifePartners: Cornelia and Lizzie have shades of this in the 1959 version with a few moments of LesYay, surprising considering their respective ages and the decade the film was made in.
* PoliceAreUseless: In the 1959 film adaption. [[spoiler: Of course the police lieutenant is useless, because ''he's'' the Bat.]]
* RapunzelHair: Cornelia in the 1959 version has waist-length hair, braided for the majority of the film (which mostly takes place at night when she's about to go to bed).