When classics get adapted or updated, I often find myself asking: What’s the added value? What do you get from Shakespeare with penguins that you don’t get better from Shakespeare straight up?

That’s the chip I had on my shoulder when I went to see “Scotland, PA,” a musical riff on “Macbeth” that opened on Wednesday at the Laura Pels Theater. It’s not as if the great tragedy hadn’t been plundered enough already; earlier “Macbeth” mash-ups include a “Macbett,” a “MacBird!” and even a “MacHomer,” in which Banquo is reconfigured as Ned Flanders.

And I already knew that this one, a world premiere commission from Roundabout Theater Company, was based on a 2001 film by William Morrissette that moves the action to the 1970s — not the most appealing era for updates. I worried the witches would be Charlie’s Angels.

But “Scotland, PA” — in which the witches, happily, are stoners instead — turns out to add some delicious value to both the original play and the film. Its smart book (by Michael Mitnick) and agreeable songs (by Adam Gwon) are often laugh-out-loud funny, something no one ever said about the version that opened in 1606. The show, directed by Lonny Price, is also quietly insightful, making piquant connections between Shakespeare’s drama of political powerlust and the consumerist mania of our own fast-food culture.