Bright Colours Only, Riverside Studios, London, ***<br></br>Debris, Latchmere Theatre, London, ***

"Are you writing the eulogy?" asked Pauline Goldsmith when she saw me with notebook and pen at the ready. We're in her living-room in the Riverside Studios, and she's a bit flustered because the loved one is in his coffin (maple, so don't put your teacup on it) and the audience have trooped in to be participants in a wake that's triply "Irish" – it's in Belfast, it seems to be happening before the funeral, and not a man jack of them (apart from Pauline) seems to have known the deceased.

Artfully exploiting the oddity of this social situation, Goldsmith trips about with twittering conviviality, making sure we are supplied with tea, whiskey and biscuits ("I meant to get Jaffa Cakes; they're not as noisy") and discoursing on the gruesome hygiene hazards of having a coffined corpse in the home and on the tricky etiquette of keeping the one who has snuffed it up to snuff visually ("You want him to look like himself... but not as though you've just come back from your holidays").

It's all very funny and oddly engaging. Just, however, when you're beginning to think that this particular funeral cortège is heading up a path well-trodden before by the likes of Evelyn Waugh, Jessica Mitford and Joe Orton, the show reveals that it was only lulling us into a sense of false security. Real deaths (some of them sectarian) begin to collide with the procession and sentences to ring out with a note of distraught spiritual-gallows humour. "God has taken my husband, my health, my love. He's trying to take my job, but he'll never take away my faith".

But, of course, there is nothing like the subject of death for bringing on defensive laughter. At the end of this touching, truthful hour of theatre, you follow the coffin into the street outside the Riverside. Which – when you reconsult the programme – makes you titter at the verb in the announcement: "Tonight Bright Colours Only was supported by Dignity Funerals."

John Donne, metaphysical poet and Dean of St Paul's, was firm on the matter. He opened one of his sermons thus: "He that will die with Christ upon Good Friday, must hear his own bell toll all Lent". By dying with Christ on Good Friday, he did not, however, mean anything remotely so literal as the grotesque project undertaken by the off-stage father in Debris, a talented two-hander by the newcomer Dennis Kelly, premiered now at the Latchmere. I happened to see this piece on Good Friday – an ideal time for taking in a show that begins with a DIY self-crucifixion job, masterminded with Heath Robinson ingenuity, by a patriarch who wants to get back at his 16-year-old son. "My son, my son, why have you forsaken me...?": the boot is on the other foot in the Bible.

Blasphemy can easily become a shallow exercise in schoolboy iconoclasm, but the blackness of the comedy is persuasively sustained in this highly promising piece. How the father wound up hoist by his own petard is gradually explained in the tortuous, overlapping and leapback narratives of two siblings, which encompass paedophile kidnap, the discovery of baby in a rubbish tip, a teenage male suckling that babe with lactated blood, and something decidedly unfunny happening on the way through the birth canal. Overwritten, but beautifully performed in Tessa Walker's production by Daniel Harcourt and Carolyn Tomkinson.

'Bright Colours Only' to 3 May (020-8237 1111)