Everyone gets thanked in the programme for Chicago, except for the people who have given it its most bankable quality: Johnny Cochrane, Marcia Clarke, Barry Scheck, Alan Dershowitz, Judges Ito and Zobel, the Eappen parents, the Menendez brothers, Louise Woodward, the villagers of Elton and OJ himself. For Chicago - the best musical in London since the National's Guys and Dolls - tells us that the criminal-justice system in the US is a branch of showbiz: "In this town, murder is a form of entertainment." Thanks to CNN and Court TV we know that in a way we didn't when it premiered back in the Seventies. Sure, Chicago is cynical, but these days, so are we.

It all started in the 1920s, when journalist Maurine Dallas Watkins wrote a play about a murder trial. In the 1970s, the team who gave us Cabaret - com- poser John Kander, lyricist Fred Ebb and director-choreographer Bob Fosse - turned it into a darkly jazzy Broadway musical. It centres round Velma, who murders her husband and lover, and Roxie, who murders her lover and lets her husband take the rap. In jail, both decide the fastest route to freedom is fame.

In Walter Bobbie's revival, we see next to nothing of Chicago and the Twenties (although we hear them loud and clear in the syncopated rhythms of the score). Everything - indeed everyone - has been stripped back to essentials: from the figure-hugging Lycra outfits to the minimal settings of chairs or ladders. A drum-beat stands for a gun shot, a dancer in a T-shirt plays the investigating officer at a homicide. This isn't naturalism. The story of Velma and Roxie is a peg, in Bobbie's dazzlingly coherent production, for a series of vaudeville routines - songs, dances and wisecracks - presented in front of an on-stage band. It's a smart retort to the blockbuster musical. Sometimes the songs are as sparely focused as a concert performance. Sometimes the dance numbers are as raunchy as a floorshow. Either way, what Chicago describes is not a world but a process.

And it does it in the language of cabaret - which brings on all those s-words: sassy, sexy, seductive, sensual, sinuous, sleazy, slinky, sophisticated and sultry. It is all these, and if it isn't quite "sensational" too, here are few reasons why. Chicago emerges as a pastiche of sexiness, and one which uses what is now a fairly familiar visual vocabulary. You wonder where Fosse would have been without bowler hats, fishnet stockings and high heels. Also, by announcing its cynicism so incontrovertibly in the opening lines ("murder, greed, corruption ... all those things we hold near and dear") the show gives itself little room to move. And finally, the two female leads are better singers than dancers, and - this being a dance musical - once or twice we applaud the effort more than the effect.

Anne Reinking loyally choreographs the show with Fosse's trademark movements, giving us the loose-limbed finger-clicking, dropped shoulders, rolling heads, bended knees, gyrating hips and hands tilted at the rims of hats. It conjures up a stressed-out languor, at times thrillingly energetic, and at times - with this company - automated and cold. It externalises sexiness to the point of brittleness.

As Velma, Ute Lemper is quite a shock. She gives a brazenly angular high-kicking performance, throaty and growling, contorting her face like a Grosz caricature. It will amaze those who know her only from her work as a singer. A giraffe may have longer legs - we could argue about that - but no giraffe has ever raised one above its head. Ruthie Henshall is more than a match as Roxie. If Lemper is all display, Hensall draws us in with a contained energy that serves her numbers superbly. Henry Goodman plays the Machiavellian, cufflink-tugging lawyer Billy Flynn with a gleamingly suave delicacy, and Nigel Planer milks the comic pathos as Roxie's nonentity husband, the potato-faced schmuck with the downturned grimace and the invisible presence. Chicago is an excellent night out. If not quite a great one.

The Irish playwright Frank McGuinness, who wrote Observe the Sons of Ulster Marching Towards the Somme, spent over a decade working on his latest play, Mutabilitie. Halfway through the three and a quarter hours, I had a fantasy that he'd mislaid his script on the way to the National and been forced to rewrite it at tremendous speed. If McGuinness had forgotten half the things that Mutabilitie is about, it would have been a better play. Set in Ireland at the end of the 16th century (after the Munster Wars) it takes on Tudor colonisation and Irish resistance, the politics of the poet Edmund Spenser and the non-politics of the playwright William Shakespeare, English Protestantism and English Catholicism, characters from Irish folk legends (King Sweney and Queen Maeve), chirpy actors from the London theatre strolling in looking for some free land, and the possibility of ancient feuds - in the end - reaching a ceasefire.

Trevor Nunn's debut production at the Cottesloe delivers this formidable mix in a static, semi-operatic style, with regular interspersions of songs. This heightens the play's laboured, lapidary and emotionally wrought qualities. McGuinness grapples with a vast and tragic subject from numerous angles, but Mutabilitie has to carry far more content than its scenes can bear. With barely any momentum of its own, it gradually sinks.

In an uncomfortably cramped traverse setting, the designer Monica Frawley gives us the entrance to Castle Kilcolman (at one end) with battlements and drawbridge, and rocks, a stream and puddles reminiscent of a castle in a computer game. Close up, the cast look too heavily costumed, like figures in a heritage museum. The Cottesloe itself is a mistake. Mutabilitie is a big play - full of big ideas, with a grand, pageant-like, pictorial quality. Either the National ought to have believed in it and put it on in the Olivier or they should have left it alone. In these hazardous conditions, Patrick Malahide plays a pursed, anguished Edmund, Anton Lesser a fierily emotional William, who emerges here as a cat-loving homo- sexual Catholic who likes to cry "miaow" when making love. As the Irish poet - the File - Aisling O'Sullivan gives another stridently forceful performance. Small consolations.

The 1976 film Bugsy Malone (libretto by Alan Parker, with words and music by Paul Williams) came out the same year as the National Youth Music Theatre was founded. Twenty-one years on, the NYNT link up with Bugsy Malone for another first: a West End run for a cast of 40 that are 16 and under. As we move through the phoney facade of the book emporium into the speakeasy, it brims with Prohibition life. In this spoof tale of mob warfare, in which the gangsters fire splurge guns filled with non-toxic foam and the radio announcer "interrupts this interruption" to bring us the latest tally of stiffs, it's a pleasure to see youngsters who can do nonchalance and tap. Too often in the West End, child actors give us high-pitched sincerity or spoilt-brat whining. But here, in the confident hands of the watchful ex-fighter Bugsy (Michael Sturges, 13), the waddling speakeasy owner Fat Sam (Paul Lowe, 16) and the elegantly crooning cleaner, Fizzy (Sean Parkins, 13), they achieve a laid-back cool.

If Bugsy treats adults as children, The Suitcase Kid treats children as adults. Jacqueline Wilson's contemporary story, adapted for Fair Play Theatre Company by Sam Snape, hits a raw nerve with its story of a 12-year-old girl called Andy (Charlie Hayes), whose parents are divorcing and taking up with new partners. Wilson firmly roots for the emotionallyvolatile Andy, moving between new homes and sharing her problems with her favourite toy, a savvily athletic rabbit called Radish (Sarah-Jane Honeywell). This brisk, humorous show is fascinatingly grown-up stuff for children.

'Chicago': Adelphi, WC2 (0171 344 0055). 'Mutabilitie': Cottesloe, SE1 (0171 928 2252), in rep to 10 Jan. 'Bugsy Malone': Queens, WC2 (0171 494 5040), to 10 Jan. 'The Suitcase Kid': The Brix, SW2 (0171 274 6460), to 20 Dec.