Okay, I couldn't find a summary anywhere, so that's what this "review" is going to be.
The Stranger is a cranky old dude with a heart of gold. He hates the world, but often donates money to those in need. Mrs. Haller is a woman who lives nearby who is similarly kind, though without the misanthropy.
She has family and friends in town, and a young boy (heir to a lot of money) falls into the river, and The Stranger jumps in and saves him.
One of Mrs. Haller's guests reveals he loves the woman and wishes to marry her. His sister approaches Mrs. Haller about this, and she reveals that she cannot marry as she was never divorced after cheating on and leaving her husband.
Mrs. Haller's guests wish to invite the Stranger to dinner as thanks for his heroics, but he refuses the invitation from the servant. Therefore, they send the man who is in love with Mrs. Haller to invite him.
When he arrives, it is revealed that he and the Stranger are old war buddies. The Stranger reveals he is so angry at the world because his wife cheated on him and left him and their five children. He hasn't seen any of them in years because their faces reminds him of their mother. His friend convinces him to join them for dinner with the caveat that he will be moving tomorrow and none of them can try to stop him.
When he gets to the dinner, he realizes that Mrs. Haller is his wife, and he leaves while she faints. She reveals all this to her guests
The Stranger's friend goes back to him and implores him to take Mrs. Haller back. He agrees to meet with her, but both agree that her infidelity has ruined all chances of a happy marriage. As they are about to leave, though, the party guests reveal that their five children have come to see them, and they all embrace, then release, then the Stranger and Mrs. Haller hug by themselves. End of show.