Sheridan Smith’s new show is more a mystery than a musical. Opening Night is based on a 1977 film by John Cassavetes that failed to attract a major US distributor. After opening briefly in LA, it vanished without trace. It’s a backstage drama about a tattooed drunk, Myrtle, who accepts the lead role in a new play which she starts to dislike. Realising her error, she tries to improve the script at the rehearsals and during preview performances ahead of the opening on Broadway. In real life, an actor who sabotaged a show like this would be fired and replaced. But never mind. This is make-believe.

Myrtle’s attempts to vandalise the script are opposed by the producer, the director and the writer, and they each moan to her in private about her behaviour. Myrtle is a tricky customer who seems to alienate everyone she meets and it’s hard to care about her squabbling over a few lines in a pretentious play. Romantically she’s torn between her leading man and her married director whom she bombards with late-night phone-calls, much to the annoyance of his wife.

Are we not all Harry Clarke, making it up as we go along, hanging on by a thread?

The strands of this cryptic story are further complicated by the arrival of a spiritual being who represents the unburied phantom of a 17-year-old theatre-goer recently killed by a car. Adding music to the ghost story makes it even harder to follow. Sometimes the characters sing their lines and sometimes they speak them normally. But are they speaking as part of the straight play or are they singing as part of the musical about the straight play? And how does the ghost fit in? Gosh it’s hard to unscramble. All the action is filmed live on stage and beamed to the audience over a number of TV screens dotted around the theatre. To help clarify the plot, the screens bear captions full of data but there isn’t enough basic information to make sense of the muddle unfolding on stage.

It’s obvious that the show’s creators, Ivo Van Hove and Rufus Wainwright, set out to reinvent the musical from scratch. But they made a couple of errors. First, they borrowed an impenetrable storyline from a film that no one’s heard of. Second, they declined to give the fans what they want from a musical, namely spectacle, glamour, beauty, style, gorgeous costumes, lavish sets, good-looking actors, and so on.

The aesthetic is wilfully shoddy and disorganised. The playing area looks like an abandoned call centre. The three male leads can’t afford a razor-blade between them. And the cheapo costumes seem to have been recovered from a suitcase abandoned at Gatwick. The whole thing looks like a production of Mother Courage performed in a mining village after a year-long strike. If you have tickets, watch the trailer of the Cassavetes movie before you leave the house. It may explain some aspects of the mystifying plot. Otherwise you’ll be in for a painful and baffling experience.

Harry Clarke is the anodyne title of a new show at the Ambassadors Theatre. The phrase ‘growing up gay in Indiana’ describes the narrative but this drama is brimful of vitality, humour, inventiveness and psychological penetration. It opens with Philip, a lonely gay kid from the midwest, who speaks with an English accent for some reason. His father is a drunken, homophobic farmer who manages to kill himself in a tractor mishap. This sets Philip free and he moves to New York, where his nerdy English accent attracts very little attention. Barely able to scrape a living from bar work, he experiments with a new persona, a loudmouthed Cockney music producer with a glamorous life and famous friends. Harry Clarke is born.

Unlike the diffident Philip, Harry is popular, successful and decisive. A chance encounter at a shoe store brings him into contact with Mark Schmidt, a wealthy in-the-closet businessman, whom Harry sets out to seduce. At the same time, Harry charms Mark’s sister and his widowed mother and he soon becomes indispensable to them all. This is an ancient story – the outsider who befriends and transforms a dysfunctional family – but this version is as fresh as a spring breeze and dazzlingly well written by David Cale. Harry never loses sight of the fact that his identity is invented and that his friendship with the Schmidts is based on a falsehood that he can never admit to. And there lies the beauty of the tale. Are we not all Harry Clarke, making it up as we go along, hanging on by a thread, desperate to maintain our fraying connections with lovers, friends and relatives?

Barry Crudup, in an immaculate performance, plays Harry like a cross between Ronnie Wood and Terry Venables. It’s a delicious irony that this fabulous yarn about a fake Londoner is about to wow London in real life.