My take on this episode of Department S is that the plot is actually straight out of The Avengers stable, an opinion which doesn't seem to be shared by anyone else, since the general opinion on the internet seems to be that this is a straightforward 1960s ITC detective/spy story.
I haven't made it explicit, but I've adopted a policy of not doing episode synopses on this blog: after all the reader who has managed to get here can also get to episode guides to most of the series I watch, elsewhere on the internet, so I'll content myself with commenting on how this episode strikes me.
The basic premise is the very Avengers one of both fascination with modern technological and chemical developments, & fear of their possible repercussions if they get out of control. In this case the powers of order, the Establishment, are contrasted with Gregory Halliday, a lone scientist who has beome concerned at the safety of the chemical weapons his department is working at. He means to demonstrate the danger of these weapons by stealing a container of enough nerve agent to kill a million people. Dr Stapeley, who is the head of the Chemical and Biological Warfare Department at Lansdowne Park, that is Halliday's own department, is portrayed as the voice of the Establishment view, which is that Britain must have these weapons as a deterrant, given that other nations have them. In the visual language of 1960s television, you can tell he represents the Establishment by his pinstripe suit & horn rim spectacles.
Mark King is posited as the voice of the radical position: that weapons these dangerous are too dangerous to possess at all, & in the last scene he is the one who resolves the situation by grabbing the vial as Halliday collapses & drops it. In his camp way he tells Stapeley that he is 'constantly' amused - Stapeley is clearly irritated by his position - but terrified by this substance.
What makes this very Avengers is that at no point is Halliday portrayed as a traitor, or baddie in any way. In fact the only way you would get a large vial of nerve agent out of a -presumably - secure research establishment is by being trusted. Halliday is a good chap, in a very Avengers way the establishment man who has gone wrong somehow. The real baddies of the piece are the men who kidnap him & try to get him to tell them, by torture, where the nerve agent is hidden. In reality the theme of this episode is that of fighting for the Establishment, which is ultimately good, placed at threat in this instance by an establishment man who hads cracked under the pressure, leaving our nation's safety at risk by the real baddies. Additionally the episode exposes a weakness in the establishment's security: I describe the container for the Otriox 5 as a vial because it is, but it's a large vial, & Halliday was able to get it out of the establishment. The placing of the real threat & the chemical aspect of this story are what put it in the Avengers stable.
This episode also allows development of Jason King's character - although even in the series of the Avengers where he was clothed by Pierre Cardin, Steed never managed the peacock finery of King. Most of the attention is on King out of the Department S staff in this episode - there are parallels of this in other episodes, though. Here he shows unexpected depth, in his vehement opposition to chemical warfare. He comes across as a rather waspish character in his criticism of Annabelle's research, but he ultimately does the majority of the actual action in this episode, ultimately taking the lead in the denouement.
Sir Anthony Hopkins plays a sweaty, dirty, unshaven & tortured Halliday to great effect. Since the rest of the Department S crew sink relatively into the background, his character actually provides an excellent foil to King's flamboyance. The scene where you can see that his right hand has been tortured provokes the viewer's sympathy for him permanently. By this time Hopkins was already Olivier's understudy, but doesn't overly outbalance the other actors in this episode.
Further stars for me in this & all Department S episodes are the 1960s-era outdoor scenes, which in this episode includes some London street scenes. Some fairly straightforward spy-fi plot devices, such as the head-on car confrontation & the obligatory underground car park scene, don't go amiss to help along the 1960s spy-fi feel of this. The device of King suddenly producing a letter from Halliday's daughter at the end is not necessary for the plot - clearly he is trying all avenues to stop Halliday destroying a million people - he might as well have gone straight on to the negotiation which follows. I like how King tells Halliday to throw the vial - Halliday clearly doesn't want to really, but has been driven to this by his concerns about chemical warfare.
In summary an enjoyable 1960s romp which tips its cap to The Avengers, while it also manages to be the vehicle for a deeper message about chemical warfare.