Jason Donovan and Stephen Sondheim are not two names you would immediately think of putting together, and I suspect many people who came to see the former - like the two sitting in front of me who left at the interval - were nonplussed by the operatic style of the latter.
But this show, which without Jason Donovan's name would no doubt have drawn a meagre audience to the Alex, is astonishingly, jaw-droppingly good.
Donovan's personal triumph is to transcend celebrity to be first among equals in a brilliantly gifted ensemble.
Well, perhaps not quite first, because if you were going to pick out just one star performance it would have to be Harriet Thorpe's as a wonderfully blowsy Mrs Lovett.
But this isn't really about stars: directed and designed by John Doyle, one-time artistic director of the Worcester Swan Theatre, it's about a company using the resources of theatre to create a strange and compelling world of horror and fragile beauty.
The ten performers not only act and sing but are also the on-stage orchestra.
Actor-musicians are familiar enough nowadays in rock musicals, but this is a classical ensemble of violin, pairs of cellos, clarinets, trumpets and piano.
The music is extraordinarily intricate (enhanced by very good stage sound) and there is not a music stand in sight, everything is played from memory.
Despite Sondheim's specific denials, Sweeney Todd here seems to be firmly in the tradition of Brecht and Weill, even sharing the same London underworld milieu as The Threepenny Opera and similarly updating the story from the 18th century to the present.
In true Brechtian style everyone is on stage throughout, performing either in character or on one instrument or another, and often both at once.
Although Jason Donovan's singing can rise to the expressionist peaks of Sondheim's score, his performance as the grisly razor-wielding serial killer is primarily one of glow-ering understatement, and he contributes some strummed guitar to the show's instrumental palette.
It would be difficult to recommend this show too highly. It's the best thing I've seen at the Alex for years, a celebration both of sophisticated Broadway music theatre and the resourcefulness of British (and Australian) actors.
* Running time: Two hours, 40 minutes. Until Saturday.