Like many of you reading this, I shall be watching the second series of The Killing this weekend, and every weekend right up until Christmas.

The reasons for doing so have been well aired in recent weeks, from the charismatic central performance of Sofie Grabol who plays the detective Sarah Lund to the atmospheric production. But at its core there is something simpler than all the complex politics that also informs the series. It's a whodunnit. And, as with any whodunnit, it would be greatly diminished if we did know whodunnit.

It's surprising we don't. We live in an age where the internet, with its blogs and random comments, and Twitter with its licence to sound off about anything, could have ruined the plot many times over. There is also Wikipedia, which is not a total stranger to spoiling a whodunnit. Look up Agatha Christie's The Mousetrap on Wikipedia, or rather don't if you ever intend seeing it at the theatre. The entry announces that there is a twist at the end. It then churlishly goes on to announce precisely what it is. The Christie estate has not been thrilled by this, and it may well have ruined an evening out for some theatregoers. But it is an exception.

The whodunnit on television and in film seems sacred, immune from the urge to tell everyone everything that is the driving passion of blogging and tweeting. And, as the latest series of The Killing being shown in Britain appeared well over a year ago in its native Denmark, it would be very easy for an ill-wisher to reveal the plot, and ruin the next few Saturday nights for most of us. But nowhere on the internet, thankfully, will you find the denouement of The Killing II.

It's a curious morality, but very welcome in an area normally devoid of such scruples. As I and many other newspaper commentators know, the usually anonymous bloggers and responders to our articles take few prisoners. They shoot from the hip, and can be vicious in their ripostes. But when it comes to the whodunnit, there seems to be a moral code, both a respect for the genre and for the viewers. Court injunctions are broken on the internet and on Twitter. Libel laws are flouted. But there remains an adherence to one of the oldest conventions in dramatic criticism: you don't give away the identity of the killer.

It restores one's faith in humanity.

The curious case of the missing racist

Bonnie Greer's opera about her appearance on Question Time with the BNP leader Nick Griffin premiered at the Royal Opera House this week. The piece by the writer and academic was called Yes, but that wasn't what the critics said.

I think it made two fundamental mistakes. Ms Greer was on stage playing herself, whereas any drama, especially if it is to last, should have a professional playing the role. Much more weirdly, this totally legitimate subject for dramatisation – the confrontation on a TV programme between a black writer and the head of the BNP – didn't portray Nick Griffin on stage at all. This strikes me as a cop-out both dramatically and politically.

If you believe, as I do, that opera is capable of embracing and illuminating any current political and emotional concern, then it seems bizarre to leave out one of the drama's leading characters. With all the expertise available at the Royal Opera House, it is surprising that Greer wasn't better advised. Besides, one could always have diminished Griffin by making him a countertenor.

Christmas viewing is getting better

We will, no doubt, be told, as every year, that Christmas TV isn't what it was. Actually, I'm not sure it ever was what it was. But the first announcements of this year's schedules are not unpromising. A two-hour Downton Abbey special will certainly be my choice on the evening of Christmas Day. There's also the return of Absolutely Fabulous to look forward to, a healthy dose of Dickens, and plenty of arts from Tosca to Darcey Bussell. It could be worth staying in front of the TV for the first time in some years.

But here's my challenge to Downton Abbey writer Julian Fellowes for the Christmas special and the next series. Have the Countess talk to her daughters. Cora seems to be a mother who rarely, if ever, exchanges a word with them. Even in the early years of the 20th century, even in aristocratic households, surely mums did chat to the girls.

d.lister@independent.co.uk / twitter.com/davidlister1