Over the course of its first four episodes, The Shadow Line has matured from an overwrought, rather confused melodrama plagued with florid verbosity into one of the BBC’s best thrillers in a very long time. In the fifth instalment, it gets even better.

BBC Two’s increasingly fascinating police drama reaches and passes its halfway stage in this episode, yet neither DI Jonah Gabriel (Chiwetel Ejiofor) nor Joseph Bede (Christopher Eccleston) appear much closer to accomplishing their respective goals than they were in the hours after the death of gangster Harvey Wratten.

Like the drugs at the heart of the late Harvey Wratton’s empire, The Shadow Line has become addictive in a slow, almost sneaky manner. After an unhurried, take-it-or-leave it start, it has insidiously built up to the point where now, three episodes in, it has become impossible to break free of its grip.

The principal problem with The Shadow Line is its identity crisis. At times, it’s a fascinating noir-ish thriller; at others, it feels like an unwieldy Shakespearian melodrama that has somehow lost its way en route to the RSC and found itself on the telly instead.

Appropriately, given that it begins with the discovery of a body, the opening episode of The Shadow Line moves at a pace that’s almost funereal, although that isn’t to say it’s boring.