Whitechapel, ITV1

You may think that Whitechapel's USP would have made a third unlikely after two successful mini-series. The first was about a modern-day copycat killer in Whitechapel who was recreating the 1888 Jack the Ripper murders, while the second was about a modern-day copycat killer who was recreating the Kray twins murders from the 1960s.

Now the East End of London may be, in estate agents' terms, a vibrant place, but there are only so many bloodthirsty periods from its history to spawn yet another modern-day psychopath.

The third season, which started last night, consists of three two-parters and the show's writers, Ben Court and Caroline Ip, have come up with an excellent wheeze to eke out an idea that already stretches our credulity - in last night's opener the Whitechapel major incident squad headed by DI Chandler (Rupert Penry-Jones) had been given the Metropolitan Police archive to keep in their basement. This one could run and run, as who knows what manner of juicy slicings and dicings lie in those box files...

Much of Whitechapel is stock; the tall, suave and cerebral DI Chandler is a fast-track detective, while Phil Davis in an ill-fitting suit is DS Miles, a career policeman who has seen it all. The maverick role is played by Steve Pemberton as Edward Buchan, a Ripperologist in the first series who just happened to have once made a documentary about the Krays in the second, and in series three he has been given a job in Chandler's squad as a researcher.

Whitechapel looks lovely and John Strickland directs efficiently, but oh dear the script is tosh

Chandler's belief is that today's detective can solve cases by learning from historical ones. So when a popular East End tailor and three of his staff were found by his female assistant bludgeoned to death in the tall Huguenot house where they made bespoke suits for today's rich Russians and Hoxton hoorays, Chandler immediately ordered Buchan to find a historical case that had striking similarities. Miles, meanwhile, suggested a more prosaic PC Plod approach of a door-to-door enquiry.

Buchan soon came up with a Victorian case of a linen draper and his family murdered in their home and found by a female relative, in a case with no obvious suspects; details of this story were dovetailed with how Chandler's investigation was going. Why did the tailor have an elaborate security system installed for a business where no cash was kept? And how did the killer enter and leave a house locked from the inside and manage to kill four people without one of them trying to defend himself or leaving a trace of DNA?

As with the earlier case, Chandler soon arrested a chap who had nothing to do with the murders, and then David Schneider entered doing a goggle-eyed turn as a taxi driver who bore a grudge against the tailor - nothing to do with an ill-fitting suit, it turned out, but the dead man was his half-brother who had received all their father's love. Yes, a bit of a sideswipe, but the story by this time was so full of melodramatic nonsense, with the local residents in the grip of a badly acted pandemonium about the killings and a murderer who prowled only in darkness and could materialise through walls, that Schneider's overacting was at least funny.

The problem with a series like this is, that however well acted by the principals (far better than the script deserves), Whitechapel cannot compare with the witty and cerebral Sherlock, the clever (if outlandish) plotting of CSI, the wit of Life on Mars or, at the other end of the crime system, the clever meddling of sexual shenanigans and slick courtroom drama in The Good Wife. Judged on its own terms, Whitechapel looks lovely and John Strickland directs efficiently, but oh dear the script is tosh and struggles to suggest any back story for the leads, while underwriting the supporting cast's roles and reducing them to exposition jockeys - “He's a coke-head, sir, with unpaid debts all over the place.”

But it's engaging enough and uchallenging Monday-night fare. If you're interested, the taxi driver is on the loose, Buchan is worried that they have the wrong man and the denouement is next week.

• Whitechapel continues on ITV1 on Mondays