Initially, Kwame Kwei-Armah's new play looks like an amusing study of racial and generational tolerance, in which a cantankerous old Caribbean Londoner establishes a rapport with his young, Polish cleaner that he can't find with his two daughters. But it ranges far wider and deeper than that, decisively transcending issues of race.

In a series of surprising turns, Let There Be Love delves into domestic violence and illness as well as gender and sexuality, dignity and death, without ever losing its sense of humour.

At times, I wished Kwei-Armah hadn't put in quite so many themes, or chosen to direct the whole thing himself. He punches up "signifi-cant" lines that an outsider might have let speak for themselves. His three actors are engaging, though, and this remains a smart and possibly noble exploration of what it takes to be human and happy.

Joseph Marcell's Alfred Morris is a crusty, ailing Grenadian, just as grumpy and likeably sardonic as the script requires him to be but somewhat less decrepit. He lives alone, refusing medication and the tentative overtures of Gemma (Sharon Duncan-Brewster), the child he dislikes least.

Gemma, though, has enough family feeling, or enough of any eye on inheriting the house, to hire Pole-on-the-make Maria (Lydia Leonard) from an agency to look in on Dad for an hour a week. Guileless and extrovert, and enduring a tough immigrant experience to rival Alfred's stories of the 1960s, Maria soon starts spending more time with her charge. Eventually we learn what he wants from her and what she is prepared to give him.

The title alludes to Alfred's belief that Nat King Cole has all the answers to life's questions. Even if he's wrong, Kwei-Armah suggests that a little love, or compassion, or forgiveness, goes a long way.

There's an underlying sweetness to the story, undercut but never quite defeated by various unsavoury or just plain boggling revelations.

The author has fun with the old tropes of racism, as Alfred accuses Maria of stealing British jobs, and Maria bridles when he mistakes her for a Czech: "You think we all look alike?" The play ends on a controversial note that transcends ethnicity, which I won't reveal but which is intelligently handled.

The acting is necessarily uneven. Alfred has to be a sprightly charmer one minute, at death's door the next. Leonard makes Maria very funny but ably accomplishes her character's swings into tragedy. Kwei-Armah has the ability to write instantly illuminating revelatory lines for characters as underwritten as Gemma, even if he can't quite integrate every element of his play. This is a flawed gem.

• Until 16 Feb (020 7328 1000).