Insignificance, Royal Theatre, Northampton

"Thank God for Joe," wrote Marilyn Monroe, the year she died. "Love, respect and thanks, Albert," said a letter found after her apparent suicide. Terry Johnson's compact drama entwining Monroe, baseball legend Joe Dimaggio (her then husband), Einstein and Joe McCarthy in a Fifties hotel bedroom is already a classic. Like Dürrenmatt's The Physicists (Einstein again) or Ronald Harwood's Taking Sides (about Denazification) it addresses, with black comedy, major events from mid-last century.

Insignificance at Northampton's Royal Theatre confirms just how significant Rupert Goold's regime is proving. Paul McCleary's broken English Professor devastated by the consequences of his own nuclear calculations (piled high on the hotel floor) and assailed by the manipulative senator (Alan Perrin), becomes - in double regard - America's conscience.

But so are Monroe, whose female intuition and monumental sharpness enchant Einstein by regaling him with his own theories, verbatim and her gum-chewing, mindless "nigger-whipping Louisiana boy" of a hubby who makes a pact - "I don't chew gum and she don't pop nembutal" - and whose adoring protectiveness sustains the drama. It was bold of Goold to cast Gina Bellman in the key role: she emerges gloriously - her gestures are an encyclopedia, the pathos of her miscarriage tangible. Steven Hartley is the perfect antithesis: James Caan rolled into Elliott Gould, a Yank who "tinks" rather than "thinks", but whose brutish vulnerability breeds animalic devotion. Four faces work brilliant overtime: this was a rapier-fine production. Good news for Northampton.

Midland theatre seems ablaze this month: there's great news from Derby too, where Stephen Edwards's sizzling staging of Sweeney Todd hits the jackpot. Set on Neil Irish's Dickensian-laden revolving stage, awesomely lit by Tina MacHugh, this - like Insignificance - could transfer to London tomorrow. Andrew Synnott's five-piece ensemble arrangement worked wonders.

Jenny Galloway's Mrs Lovett gave the giant of a performance: her face, too, is a picture, charting the hair-raising fortunes of Fleet Street's terrible piemaker, who ends up in hot trouble; "by the sea" was just perfect. Craig Purnell's Tobias Ragg set the evening alight. He travels the gamut from cheerful Touchstone to Uriah Heep to Poor Tom: real range. But Edwards had amassed a super team: Lyndon Terracini's Sweeney lacks the notes but fixed with his Richard Burton glare and Lee Marvin croon. Philip Ramm's Pirelli was a hoot; Martin Callaghan's Beadle finessed; and Ray Gabbard's Anthony Hope, the best sung. A treat from start to finish.