There is no such thing as a new detective drama.

The format was invented by Wilkie Collins, developed by Arthur Conan Doyle and mastered by Agatha Christie, and in the 20th and 21st centuries, it's been tweaked here and there but remained essentially the same. Look carefully at any detective show and you can always see something of Wilkie, Arthur or Agatha.

Lewis (STV, Friday, 9pm) is now a spin-off of a spin-off of a spin-off because it started with Inspector Morse with Lewis as his sidekick, then it became Lewis with Hathaway as his sidekick, and now it's Hathaway with Lewis as a kind of sidekick. Lewis is now the man who never retires, a consultant, a nitpicker, a pointer-outer of problems.

The bonus of this change is that it's given a reliable old format a vitamin boost by breaking up the chumminess of Lewis and Hathaway and introducing a new awkwardness in their relationship. At the end of the last series, Lewis had retired but has now come back and Hathaway, who'd been running the show, is not happy about it. And can you blame him: a star never wants to rejoin the

chorus line.

One of the other changes is that Hathaway has a new DS, Lizzie Maddox, played by Angela Griffin, who does at least help to break up the all-white face of detective fiction. It's a little strange that in real life, well-off, white, middle-class people are extremely unlikely to be the victims of crime but in detective fiction they have the threat of murder hovering over them at all times. Perhaps it's one of the less pleasant consequences of the rules of detective fiction being written by well-off, white, middle-class people such as Wilkie, Arthur and Agatha.

Lewis does make a slight effort to address this, but otherwise it sticks to the traditions of whodunnit. It also has a reliable cast: Lewis, played by Kevin Whately who, as he widens and sags, is starting to resemble a Bake-Off showstopper that's been left in the oven too long, and Laurence Fox, who frowned once in 1980 and, as his mother surely warned him it would, the wind changed and his face was stuck like that.

In last night's episode, there was also another delightful addition to the cast: Anna Carteret, who you may remember (if you had nothing better to do in the 1980s) played Inspector Kate Longton in Juliet Bravo. That programme, 30 years ago, was the first British police series to feature a woman in the lead, but how funny and sad that it is still one of the very few innovations in the old format of detective fiction. Lewis is perfectly good, but it does slightly make me crave more innovation. Wilkie, Arthur and Agatha may have written the rules, but doesn't television desperately need someone else to come along and break them?