A truly excellent example of the British thriller genre produced on modest budgets in the late 50's and early'60s. Nigel Patrick goes against type from his usual suave, raffish and urbane gent about town - although he's still a fairly suave and urbane Detective Inspector here, albeit a very doggedly determined one.
DI Johnnoe prefers the old tried and tested methods of crime busting, including associating with known criminals as snouts. His boss (Harry Andrews) , backed up by the weasely and ambitious Smythe (Allan Cuthbertson), is keen to do away with these old habits and trouble lies in store when Johnnoe persists after one of his snouts is bumped off.
The supporting cast is magnificent, with a roll call of superb Brit actors, many of whom were just emerging into fame at the time. In particular, Frank Finlay, Colin Blakely (both outstanding), also Derren Nesbitt, George Sewell, Brian Wilde, to name but a few. Even Peter Bowles gets an early (uncredited) turn as the mysteriously-named "Peter the Pole".
The final punch up is a little chaotic, but it doesn't detract from the overall narrative of the film which starts as a detective vs bad guys story, but soon evolves into a much deeper and engaging theme. Thoroughly recommended.