Wonder wall

"So what do we know?" asks someone near the start of Murphy's Law, which is occupying our peaktime attention for three consecutive nights.

The answer, I'm afraid, is very little. I was paying attention, honestly, but this drama moves in such mysterious circles that you need an honours degree in obscure thrillers to follow what's going on.

This can be very annoying. If you are going to devote three hours of your life to a TV programme, then you want a few explanations.

This three-parter begins with an intriguing situation and then one of those "40 hours earlier" captions which have come to seem like a lazy way for a writer to draw us into the plot.

Admittedly, Murphy's Law is very well done, a class act in the world of grim, gritty cop shows. James Nesbitt hides his natural charm behind a scowl and an amazing moustache as an undercover cop whose assignments never go to plan. This time, he's lost one of the team - Kim Goodall - who's been working undercover in Norfolk.

The plot thickens like lumpy gravy on Tuesday and Wednesday, although it's doubtful if the BBC budget will extend to buying more light bulbs to replace the ones that are broken, making the experience resemble a night in the Black Hole of Calcutta.

Things are much brighter in The Sarah Jane Adventures, a Doctor Who spin-off aimed at the younger market but which adults will find just as appealing.

The Time Lord's one-time travelling companion is a journalist who investigates aliens, thus removing much of the surprise element from the plot. We can anticipate exactly what she's going to be doing each week, it's just a matter of waiting to see which slippery, slimy creature from outer space turns up.

Today, it's the Slitheen. These green aliens moved into Downing Street in a previous Doctor Who adventure. Now they want to drain the Earth of its energy or some such nonsense, which involves taking over the bodies of teachers at the school attended by Sarah Jane's son.

The Slitheen give themselves away by their farting. That's right, they're the universe's first farting aliens.

School assembly is a hoot as the headmaster and teachers recreate the famous baked beans campfire scene from Blazing Saddles.

It's good to see Elisabeth Sladen back as Sarah Jane, looking hardly a day older than when she last flew in the Tardis. The whole thing is fast, furious and funny. When was the last time you heard anyone say, "What are we going to do - stand there and throw pickled eggs at them?".

They could build a big wall instead. A very big wall in the case of the Chinese five centuries ago. The idea was to keep out the Mongol tribes, descendants of Genghis Khan out for a spot of invasion. The result was the greatest defensive structure ever built - the Great Wall of China.

Two hours devoted to a wall might seem a little excessive, but the story is a fascinating one and the presentation employs more lavish reconstructions - grand palaces, colourful costumes and many extras for the battle scenes - than most docu-dramas.

The Chinese Emperor realised something needed to done when the invading Mongols reached the suburbs of Beijing. This interferred with his daily duties of taking drugs and having sex with as many concubines as possible.

Historian Professor David Spindler, one of the experts acting as our guide, goes off to the old border town of Xiakov to view the old defences - high earth walls - and you can understand why the Mongols were able to get in so easily.

The way to keep them out, the Chinese decided, was to build a single wall with layer on layer of fortresses adding up to a vast defensive network of brick and stone wall, towers, storehouses, arsenals, beacons and refuges.

The result was a 3,000-mile-long fortress built on top of a natural barrier with no gaps and no weak links. Anne Robinson, take note.