CSC, best known for its summer productions of Shakespeare on Boston Common, is making its indoor debut with “The Last Will,” which is the final installment of Brustein’s Shakespeare trilogy, following “The English Channel” (the Bard’s affair with the Dark Lady of the sonnets) and “Mortal Terror” (King James I, “Macbeth,” and the Gunpowder Plot).

The set at the Modern, designed by Eric Levenson, is a three-level affair made of metal. CSC artistic director Steven Maler says it “feels like a deconstructed Globe Theatre.” And that’s appropriate for what Maler calls “a dream play, a memory play. It all could take place in three minutes inside Shakespeare’s head in the last moments of his life.”

But the play is also, to a degree, historical. “You don’t alter the facts,” says Brustein, 85, who founded both the American Repertory Theater and Yale Repertory Theatre in New Haven, and is now a distinguished scholar in residence at Suffolk University. “You use the facts, and then you speculate on top of them, to a certain extent. I have speculated based on what Shakespeare says in his plays and what he says in his poems.”

In his 2009 book, “The Tainted Muse,” Brustein adds, he made the case that “there are certain urgent issues that came up over and over in Shakespeare’s plays — for example, infidelity on the part of wives. And even if they proved not to be true, as in the case of ‘The Winter’s Tale,’ these issues were repeated so often, and with such fervency, that they came out of something real in Shakespeare’s life. Especially the sonnets with regard to the Dark Lady. So I found certain things like that throughout the plays and assumed that they happened in Shakespeare’s life and speculated about how.”

In “The Last Will,” Brustein speculates that Will Shakespeare (Allyn Burrows), having returned home to Stratford and to Anne (Brooke Adams), emulates Leontes in “The Winter’s Tale” by entertaining doubts about the legitimacy of his twins, Hamnet, who died in 1596, and Judith (Stacy Fischer). Anne’s handkerchief, discovered in suspect circumstances, recalls Desdemona and the jealousy of her husband, Othello. The play also echoes, darkly, “King Lear,” as Will tries to decide which of his surviving offspring — Judith, who wants to marry a man he doesn’t approve of, or Susanna (Merritt Janson) — loves him most. He’s getting advice from one of his greatest actors, Richard Burbage (Jeremiah Kissel), who wants him to come back to London, and from his attorney, Francis Collins (Billy Meleady), who is executing the Bard’s last will.

But Will himself isn’t necessarily of sound mind. “In the sonnets he talks about having contracted syphilis from the Dark Lady,” says Brustein. “And I simply assumed that syphilis had run its course and become paresis by the end of his life, which is a deterioration of the brain, and that he was going mad and confusing himself with his own plays. And in fact that was the truth. He often confused himself with his own plays, because he wrote out of his life, as I believe.”

So what was Shakespeare’s life like with Anne? “The notion that Shakespeare left his wife the second-best bed has always puzzled people and alerted them to something going on in that marriage,” Brustein explains. “Plus the fact that he was eight years younger than she was and that he married her because of an unwanted pregnancy.”

In “The Last Will,” he says, “It’s a relationship that is mended as the play goes on. He gradually reaches a reconciliation with her, which is what he’s been after. But he then unfortunately falls victim to his disease. There’s a love story in there all right. It’s complicated, like most love stories.”