This is a strange, antediluvian pleasantry (old-fashioned even in 1974), resembling nothing so much as one of those Francis Durbridge serials where everyone is a suspect, everyone has a secret and everyone lies like mad.
A businessman is shot, and suspicion falls on his 'friends' (all parasitical hangers-on), 'secretary' (mistress) and his estranged son (John Thaw, just prior to his Sweeney superstardom). A gritty detective (Glyn Owen) and an enigmatic MI5 man (Peter Sallis) investigate, and soon link the crime to the murders of various shady Chicago characters in the UK. The suspects are slain at regular intervals, until the final unmasking of the villain and a somewhat perfunctory climax.
Absolutely nothing wrong with this, and it is great fun for those who like Paul Temple, or who prefer the slower pace of 70s television, and a whodunnit spread over six episodes. It is certainly not cutting edge drama, and is a surprising product of the pen of Ian Kennedy Martin, who usually produced somewhat more sophisticated material than this.
The one really hard thing for the modern viewer to cope with is the dreadful CSO, especially as for some reason a large part of this drama seems to involve deep discussions between car drivers and their passengers, all outlined by a terrible blue line, and with the background remaining stubbornly at the same perspective as the camera zooms in and out on the characters. It couldn't have looked good even then.