The Changeling is Thomas Middleton and William Rowley's 1622 masterpiece. In my opinion, it is the greatest tragedy in English outside of Shakespeare, though I probably haven't read enough to make such judgements. Right up there with Lear and MacBeth and Hamlet, if you ask me. In a nutshell - skip this paragraph if you don't want a plot spoiler - it's the story of a woman, Beatrice-Joanna, who gets her servant Deflores, who she (thinks she) loathes and who is in love with her, to murder her fiancee so she can marry someone else, only to discover he expects sex as a payment. Eventual upshot is they both die. All sorts of complicated things going on concerning madness, identity, sexual desire, self-knowledge, sin, purity, love, hate, wanking in gloves: all that good stuff. Here's an essay discussing the relationship between sub- and main plot in the play.

Here is the strangest thing about The Changeling: it is a play fundamentally concerned with madness, and yet contains an asylum which houses no named mad characters. Rather, those we meet in Alibius’ institution are all thoroughly sane. Those who live in the castle, on the other hand, come much closer to crossing the line between reason and insanity.

This, then, is one of the principle roles of the subplot in The Changeling: it makes us consider how we define madness, and how much we rely on the labels given us by society to make such decisions. There is a deep irony in the contrast between Alibius’ efforts to cure men who are already well and the unchecked descent into unreason of Deflores and Beatrice-Joanna. It asks a question which has become a cliché: who is really mad? And how can we tell?

Different kinds of insanity emerge during the play. There is the paradoxically cogent and self-aware psychosexual obsession of Deflores, the hysteria and gradual separation from society of Beatrice-Joanna, and the relatively straightforward delirium of the inmates at the asylum. The most damaging forms are shown to be those which retain a shred of reason and self-awareness, rather than the blissful ignorance of the ‘madmen’ and ‘fools’; without the counterpoint of the subplot, the horrific drift into madness of the main plot would be far less effective.

This is partly due, also, to the straightforward presentation of madness provided in the subplot. For whilst it is true that there are no major characters in this story who are insane, there is nevertheless a background of genuine insanity, shown most graphically in the macabre dance of the inmates at the stage direction

madmen above, some as birds, others as beasts
and in the horrible cries of the imprisoned madmen which prompt Lollio to come after them with his whip:
Catch there, catch the last couple in hell!
which surely refers at some level to Deflores and Beatrice-Joanna as a hellish couple, and also fated to die together. Thus insanity is portrayed as being very close to prophecy; and the madmen have a mysterious choric role to play, which elevates them from mere imbeciles but makes their lunacy all the more malign and fearful, therefore colouring our interpretation of the fall of Beatrice-Joanna and Deflores in the same way. As M.C. Bradbrook suggests, ‘the old worn pun on the contemporary game of barley break gains in horror when Deflores echoes it to Alsemero in the final scene’:
I coupled with your mate
At barley-break: now we are left in hell.

The echoes between the two plots are not merely verbal. Simply in terms of narrative alone the two have a great deal in common. Both concern a woman’s virginity under unwanted attack; both concern jealous (in one case with good reason, in the other not) husbands; both concern a woman with three suitors (Lollio is not really a serious suitor, but rather, we can surmise from his coarse, sexually charged language, a generally lascivious figure who would woo any woman and is not interested in Isabella in particular - until he thinks she is available). Most interesting, perhaps, is the comparison (as drawn by William Empson) between Deflores’ demand for sexual favours in return for committing the crime and Beatrice-Joanna’s reaction, and Lolillo’s demand for the same as the price of his silence over Isabella’s encounter with Antonio. Whereas Joanna is devastated by her agent’s demands (in perhaps the most powerful scene in the play) and eventually acquiesces, Isabella’s reaction is a brisk silent, mute,
Mute as a statue, or his injunction
For me enjoying, shall be to cut thy throat.
So on the one hand sex is given as payment for a murder ; on the other, murder is threatened as retribution for a demand for sex. This opposition illuminates our understanding of both relationships, in particular in terms of the relative balance of power. Deflores seems more powerful and more frightening because of how easily his counterpart is dismissed.

Given all these similarities between the sub- and main plot, we are prompted to ask the question ‘why are they different?’ the answer, of course, is that one contains a murder and the other does not, that one ends tragically and the other does not, that one shows us a woman gradually coming to recognise herself in the man she loathes and the other shows us a drama on a much smaller scale. This all sounds obvious - but on the stage having a less intense counterpoint acts as contrast, echo and relief, all at the same time. This is peculiarly true for The Changeling because it is so unique in the way the two plots are extremely closely bound thematically and stylistically, but remain more or less separate stories until the final act. Perhaps the roughly contemporary play closest to it in this respect is King Lear.

There is a very strong parallel between the subplot in The Changeling and the subplot in Lear. The Lear subplot is admittedly closer to the main plot in narrative terms than Middleton and Rowley’s work, but much else is shared. Both use the dramatic conceit of pretend madmen as an ironic juxtaposition with the truly unstable, and show us a world in which status is at least as important as state of mind when deciding who is mad and who is sane. Edgar/Poor Tom and Antonio, the Changeling, have a great deal in common. Both pretend to be mad to achieve an end: both find themselves, a some level or other, enjoying the act for its own sake.

This parallel may be taken further. There is a powerful similarity, too, in the way the subplot of each play is used as a comic contrast with the main storyline, and in the way in which they make us complicit in the horror by making us laugh cruelly, at what seem like inappropriate moments. Gloucester’s ‘fall’ from the ‘cliffs of Dover’ - when in fact he simply falls flat on his face at ground level - is in many ways a tragic moment, but cannot fail to look absurd: just as in this moment the unprepared audience may laugh and then feel ashamed of laughing, so in The Changeling may we react to the apparent absurdities of the madmen. As William Empson pointed out, one reason why the subplot has often been criticised is that it makes people uncomfortable: it pushes us towards a seventeenth century fear and loathing for the mentally ill, at least in the context of the play.

This ability of the playwrights to make us re-examine our own consciences and question how we ought to be reacting is greatly augmented by the presence in both plays of a malcontent. Edmond and Deflores are similar characters in the way they can both persuade us to slightly fall for them despite their evil because of heir status as the only characters who are truly honest with themselves. As Edmond happily admits he was born evil, Deflores never claims he isn’t interested in money as well as the affections of Beatrice-Joanna, always accepts his own ugliness, and does not pretend any qualms over the murder of Piracquo. They are similarly perceptive in their views of the rest of the world, and when this is combined with a propensity for asides to the audience, as it is in both cases, it is hard not to root for them a little. In The Changeling, this is greatly helped by the existence of the subplot, since it gives a wider human world which demonstrates the accuracy of Deflores’ cynicism away from his own story. We may well be seduced by his unstinting honesty and intelligence.

Very close to this idea of feeling guilty and therefore implicated is the way both subplots make use of embarrassment. At the most horrific moments our instinctive reaction may be to look away. The blinding of Gloucester is the supreme example of this, but the madmen’s dance and, to a lesser extent, Lollio’s crude sexual punning may have a similar effect. Such reactions make us more completely involved in the play since they implicate us in the action: when we return to the main plot, our attention is refocused since we have been forced to consider our own response. Thus the subplot may at once act as a welcome change in pace and tone from the relentless march of the main plot towards its tragic conclusion, and yet also intensify the tragedy and suggest different possible reactions or underlying themes.

Thematically, there is a strong parallel between the audience’s relationship with the subplot and Beatrice-Joanna’s with Deflores. Just as we are simultaneously appalled and amused by the story set in the madhouse because of its characteristics of horror on the one hand and humour on the other, so she is both repelled by and drawn to her servant, at some stages simultaneously:

I’m forced to love thee now
‘Cause thou provid’st so carefully for my honour.

This compelling aspect of the story - Joanna’s gradual realisation that, despite their superficial differences, she is no better than Deflores - is thus made still more affecting. Also, because we have been forced by the subplot to reconsider our definitions of madness and sanity already,we will also probably wonder if we have anything in common with the villains of the piece. This disturbing avenue would be inaccessible without the groundwork laid down by the subplot.

Perhaps the strangest aspect of the madhouse story is the way it gives us a kind of second Beatrice-Joanna. As Richard Dutton points out in his essay in the OUP Middleton, the text ambiguously refers to a dead ‘fellow’, who might be either Beatrice-Joanna’s mother or sister: this is strangely echoed both by the way she is always referred to simply as Joanna until act 5 and by the existence in Isabella of a purer version: where Beatrice-Joanna is vengeful, she is calm; where Beatrice-Joanna loses her virginity before marriage to a servant she is (ultimately) chaste. Beatrice-Joanna’s ‘doubleness’ - her exterior beauty and initial place in society versus her true nature and eventual fate - is nicely emphasised by this contrast.

It seems extraordinary that the subplot has ever been critically considered hugely inferior to the main story and a burden on the play. WIthout it, the main plot is untethered. The secondary storyline is essential to provide a kind of commentary and alternative, in various different ways, to the world of the castle, and is successful in it’s own right, too. Without the subplot, The Changeling would be a grisly morality tale, straightforwardly didactic and memorable only for the isolated dramatic power of a few scenes. With its power to act as both counterpoint and parallel, the play is transformed into a work of art capable of forcing the audience to examine their own lives in a new and troubling light.