Arms and the Man is an anti-war play by George Bernard Shaw.  It is one of his first plays, and the title is taken from the first line of Virgil's epic poem the Aeneid ("Arms, and the man I sing..." Book I, line 1.)  There is a full summary here at enotes but the gist of the story is this:

It is 1885, and the war between Serbia and Bulgaria is raging.  Raina Petkoff, a young Bulgarian woman, is engaged to marry a cavalry officer named Major Sergius Saranoff.  A battle takes place close by Raina's home, and she hears that her fiance has distinguished himself in it. To escape the fighting, a Serbian soldier (who is actually a Swiss mercenary) climbs into Raina's window.  Raina, hides the soldier, even when she is questioned about it.  She learns that the soldier, Captain Bluntschli, does not carry weapons, and really despises war.  He carries food instead.  The mercenary describes the foolish actions of earnest soldiers, such as those taken by Sergius today.  Bluntshli is merely interested in keeping himself alive, not in gaining glory on the battlefield by killing and wounding other people.

Raina agrees to hide the mercenary, and he falls asleep on her bed.  Raina's mother, Catherine, finds him, but the family decides to let him hide in their house.

A few months later, Raina's father, a major in the Bulgarian army, comes home and tells the family that the war is over.  Major Petkoff also says that Raina's fiance, Sergius, is not a good soldier.  Shortly thereafter Sergius arrives, and says that he is leaving the army.  He talks about a Swiss officer who had, he says, cheated him in a deal involving a horse, and also who had been sheltered by two Bulgarian ladies during the house-to-house fighting in the city.  Catherine and Raina act suitably shocked.

When they are alone, Raina and Sergius have a syrupy-sweet reunion.  But when Raina is out of the room, Sergius renews his lewd advances to Louka, the maid.  Louka taunts Sergius, saying that Raina loves someone else more than she loves him.  Raina and her mother are almost caught out in their deception, for Raina's father asks for the coat that they had given to Captain Bluntschli.  They try to cover it up, but Major Petkoff discovers them.  Bluntschli offers to help Major Petkoff with military administrative matters, and Major Petkoff invites him to stay.

Major Petkoff wants his old coat, which had formerly been in the possession of Captain Bluntschli.  Catherine has spirited the coat, behind her husband's back, into a closet where he had previously looked for it.  He goes back and finds it there, thinking he had made a mistake.  Bluntschli has made up excellent orders for the Bulgarian troops, and Sergius is taken up with expediting them.  He takes Major Petkoff with him, and Bluntschli and Raina talk.  She tells him she had left a note in the pocket of the coat she had lent him, but he never discovered it.  Her father is now wearing the coat.

Sergius finds out from Louka about Raina's feelings for Bluntschli, and he challenges him to a duel.  Raina prevents it, by telling Sergius she knows about his feelings for Louka.  Major Petkoff returns, and Raina surreptitiously takes the photo out of the pocket of his coat, not knowing that her father has already seen it.  The Major looks for it, and, not finding it, causes everyone to now tell the truth.  Sergius becomes engaged to Louka, and Raina is now set to marry Bluntschli.  The play ends happily.