Writer Steven Moffat has created a monster – and the company does the rest on probably the funniest night you will ever enjoy at Chichester Festival Theatre. Brilliantly written, brilliantly cast and brilliantly performed, this is yet another seriously (well, in this case hilariously) first-class moment in the CFT’s increasingly remarkable 60th anniversary season. You’d hope the theatre would do itself proud in this landmark year – and boy, is it doing so. And just think, this is a play we could easily have lost. How lucky we are that CFT artistic director Daniel Evans kept faith with a piece which first should have seen the light of day during the abandoned summer season of 2020. Moffat’s starting point is that weird politeness that we tend to have as Brits (or in reality, a politeness we like to think we have). His central couple Peter and Debbie (Reece Shearsmith and Amanda Abbington) have it in spades, making them easy prey for Frances Barber’s Elsa, an outrageously over-the-top American widow who rather one-sidedly befriends them on a cruise. When she suggests they exchange email addresses, the Brits are far too polite to say no. Besides, their easy assumption is that absolutely nothing will come of it. Except, of course, it does. Soon enough Elsa announces her arrival into their rather fractured household, but not before Debbie rather belatedly googles Elsa and discovers that, in all likelihood, they are opening their home to a serial killer. And do they cling to that politeness? Of course, they do. Do they find a way to unfriend her? Absolutely not. Her impact is total – and totally unexpected as the family find themselves coming together in ways they could never have anticipated. And therein lies much of the comedy, particularly in the transformation of their teenage children. Maddie Holliday is terrific as resentful Rosie; Gabriel Howell is superb as remote Alex. Moffat wonderfully captures them both – good kids but utterly disconnected. Howell’s Alex is hilariously observed – and hilariously delivered. Meanwhile the tensions are ratcheting up for Peter and Debbie, comic acting at its finest from Shearsmith and Abbington. A marvellous scene sees them rehearse how they are going to boot Elsa out; what actually happens then follows. Frances Barber is superb as Elsa, so adept at turning any suggestion that she might have killed anyone into a ghastly discourtesy. Even worse, Peter and Debbie start getting used to having her around. Meanwhile Michael Simkins, in a lovely performance as The Neighbour, is pursuing his own completely trivial agenda. The collision of it all is blissfully, painfully funny – fine writing colliding with fine timing and fine directing from Mark Gatiss. Who would have thought that the two words “Can I?” could be quite so comical as Shearsmith squirmingly tries to protect a gastrically-challenged policeman (Marcus Onilude) without letting the cat out of the bag. Every now and again we get a truly special night at the theatre. We certainly did tonight. And the ending? Fantastic.