Willy Russell's Blood Brothers is musical theatre's equivalent of The Mousetrap in that it's been running for decades and, having been seen and loved by millions, is in some ways beyond criticism. Though I'll certainly give it a try.

For a musical it doesn't have any great songs and the plot can best be described as hokey - 'mawkishly sentimental, contrived' for those who don't speak American - but, somehow, it works. And not just in the perfunctory, mechanical way it should. The end result is, quite mysteriously, absolutely glorious.

Set in Liverpool in the '70s, the it tells the story of Mrs Johnstone (Lyn Paul) a poor Catholic woman who has seven kids and another on the way. Except the other, according to her gynaecologist, is actually twins.

She had budgeted to just about scrape by when the new baby arrives, but her scrimping certainly won't extend to feeding two mouths.  She has a job cleaning house of local posho Mrs Lyons (Paula Tappenden) who can't have kids herself and can't adopt as her husband is dead set against raising a child who doesn't share his DNA. So Mrs Johnstone has one too many kids, Mrs Lyons one too few and, fortuitously, her husband is out of the country for nine months...

Lyons persuades Mrs Johnstone to hand over one of her children. The action then moves on seven years and Mickey (Sean Jones), the son who stayed with his natural mother, fortuitously becomes best friends with neighbourhood toff Eddie (Simon Willmont).

They innocently play together, pronouncing themselves blood brothers, little realising how apt that term is. But as they grown older their lives diverge on different tracks determined by class until they both fall in love with the same childhood sweetheart Linda (Anna Sambrooks).

From adults playing kids (although brilliantly done) to a mother crying as an evil posh lady takes one of her babies, the show is loaded with scenes that the cynic in me should heartily abhor. But when Johnstone allows Mrs Lyons to do this, and turns away as she doesn't want to see the act itself, my rebellious eyes started to well up with tears.

The show has one big song in Tell Me It's Not True which could, conceivably, make it onto a Babs/Shirley showtune album, but nothing else is really that memorable. Apart from a song, sung at various point throughout the evening, which likens various situations in the characters' lives to being "just like Marilyn Monroe" (which is fine when dealing with a boy's first crush, but a bit strained when describing the experience of going to jail).

And let's not even start on its examination of class (poor=heart of gold, rich= callous, cold and manipulative).

But still, despite having every hurdle known to man thrown at it, it's a marvellous show. Lyn Paul perfectly inhabits the role of the star-crossed mother whose maternal warmth can change in an instant to the fierceness of a lioness protecting her kids. Jones and Willmont both excel playing their characters as kids and later the adults they become.

It might even be a hidden stage direction but every performance of Blood Brothers ends with the audience giving it a standing ovation. It happened tonight, and it will happen when you see it. The show deserves it in spades.

Blood Brothers continues until Saturday 14.

For more information and to book tickets view:
www.ambassadortickets.com/Theatre-Royal-Brighton