Thomas Otway is perhaps most famous for his 1682 tragedy Venice Preserv'd, but he had already gained considerable fame two years earlier with a peculiar little play called The Orphan.
Like Venice Preserv'd, Otway's The Orphan depicts flawed yet noble characters vacillating back and forth between incredible heroics and despicable villainies. In the first act, we meet the two sons of Acasto, Castalio and Polydore. Both of them are in love with the titular orphan, Monimia. She is the daughter of their father's dead friend, and Acasto has welcomed her into his household and continued to raise her as his own. So, yes, it's creepy and quasi-incestuous.
Castalio and Polydore were just out hunting (where both were performing feats of bravery) when they disclose their loves to one another. Castalio, the elder, loved Monimia first, and he claims her as his right. (Umm... I'm not sure that's how love works.) Polydore objects to his brother's love. Surely Castalio would not marry the love of his own dear brother? At the mention of marriage, Castalio changes his tune:
No! were she all desire could wish, as fair
As would the vainest of her Sex be thought,
With Wealth beyond what Woman pride could waste,
She should not cheat me of my Freedom. Marry?
Castalio agrees to step aside and allow his brother to court Monimia, so long as he does nothing dishonorable. Castalio's own honor, however, might be called into question when he introduces Polydore to Monimia and then promptly disappears. Monimia rejects Polydore's suit in no uncertain terms, and Polydore responds with a long, misogynist rant. At this point, neither one of the brothers is likely to have our respect.
In the second act, however, we learn that Castalio plans to marry Monimia after all, though he's more than a little embarrassed by the strength of his love for her. He also recognizes that he should not have played a trick on his brother by getting him to profess love to an already engaged woman. No one, Castalio claims, could love Monimia the way he does:
I am a doating honest Slave, design'd
For Bondage, Marriage bonds, which I've sworn
To wear: It is the onely thing I e're
Hid from his knowledge; and he'l sure forgive
The first Transgression of a wretched Friend
Betray'd to Love and all its little follies.
Castalio makes up with Monimia and promises to marry her. When they meet again in Act III, it is to plan the consummation of their secret marriage. She gives him a signal with specific instructions so as not to wake his father who sleeps in the next chamber:
Just three soft stroakes upon the Chamber door.
And at that Signal you shall gain Admittance:
But speak not the least word; for if you should,
'Tis surely heard and all will be betrayed.
Polydore, not knowing the two lovebirds are married, overhears this bit of information. Incensed that his brother has betrayed him and the woman he loves has rejected him--presumably for the illicit pleasures of a false brother--he comes up with a plan. He will get his page to distract Castalio, then he himself will approach in the dead of night, giving three soft knocks on her door. Since all lights will be out and Monimia has already specified that there can be no talking, she will welcome him into her bed thinking he is his brother Castalio!
The old bed trick appears in earlier plays, such as Shakespeare's All's Well That Ends Well and Measure for Measure, but in those cases it is virtuous women tricking scoundrels into sleeping with their own jilted brides. In The Orphan, the bed trick becomes a scene of horrifying rape. What is more, when Castalio shows up just as Monimia is getting busy with his brother, her maid sends him away, thinking he is Polydore come to hopelessly plead his case once again.
In the fourth act, Castalio and Monimia both ignorantly accuse each other of breaking faith. He wonders why she locked him out in the cold after their wedding ceremony, and she wonders why he left so hurriedly after sex. Polydore, the cause of all this trouble, feels guilty, but sees no way out of his crimes other than despair. At the close of the act he laments:
That's well contriv'd! then thus let's go together
Full of guilt, distracted where to roam,
Like the first Wretched Pair expell'd their Paradise.
Let's find some place where Adders nest in Winter,
Loathsome and Venemous; Where poisons hang
Like Gums against the Walls; where Witches meet
By night and feed upon some pamper'd Imp,
Fat with the Blood of Babes: There we'll inhabit,
And live up to the height of desperation.
In the final act, people run upon swords, take poison, stab themselves, and generally come to bad ends. We are inclined to agree with Polydore when he complains to his brother:
Hadst thou, Castalio, us'd me like a Friend,
This ne're had happen'd; hadst thou let me know
Thy Marriage, we had all now met in joy....