Given current events, any insight into the way in which Russia operates on the world stage is worth paying attention to, so the timing of Hunting the KGB Killers (Channel 4) feels apt. This is a rigorous and gripping documentary that tells the story of what happened to former KGB officer Alexander Litvinenko, poisoned with one million times the lethal dose of polonium in London in 2006. It is all but certain that the highest levels of Russian government were involved, according to the British courts, which makes this film as alarming as it is fascinating.

Many of those involved talk on screen here for the first time. We hear from Litvinenko’s widow, Marina, who speaks with a quiet sense of loss and a furiously dignified sense of justice, as well as his son, Anatoly. The then foreign secretary Margaret Beckett offers her perspective on what it has done for Anglo-Russian relations. The detectives who worked on the case assess its impossibilities with professionalism and an occasional flash of grim humour.

Hunting the KGB Killers operates within the vogue-ish documentary parameters of bombastic strings and blurry reenactments, which belong to school educational films and low-budget Channel 5 programmes about decadent pop stars. Nevertheless, the extraordinary, troubling story whizzes by – leaving one with the feeling that, by condensing such a vast story down to a robust 90 minutes, there’s plenty that must have been left out.

It starts from the moment Scotland Yard detectives are sent to a hospital to speak to a gravely ill man named “Edwin Carter”. At first, they admit they have trouble believing that he is a former KGB agent likely to have been poisoned, but at this point Litvinenko is still able to talk, and he does, extensively. We see flashback footage of the press conference he called in Moscow in 1998, to expose state corruption. Most shockingly, there is footage of him accusing Putin of ordering the assassination of journalist Anna Politkovskaya; at the time of making the charge, according to the voiceover, the polonium was already in his system. It’s chilling.

Hunting the KGB Killers is powerful on two levels. It works as a thriller, such is the complex, frustrating nature of the work the detectives had to do in order to try to crack the case. At one point, having been sent to Russia to speak to the two prime suspects, one officer says he thinks the tea they were offered was poisoned, just enough to physically weaken them and affect their ability to conduct their investigation. They had upset stomachs, but it didn’t stop them. “We just had to do it in short bursts,” says DI Brian Tarpey, wryly. There are other moments so outrageous that they’d be too much in a fictional spy movie, such as the questioning of one suspect, Dmitry Kovtun, carried out in Russian as Met detectives had been told he spoke no English. At the end of the session, Kovtun wishes them luck – in English. The tapes mysteriously never made it back to the UK.

But it also works as a story about people and family and loss, never mawkish, but moving. “It’s about how to be human,” says Marina. Her persistence in ensuring a public inquiry despite Theresa May’s apparent reluctance is staggeringly brave. “I believe he’ll be able to see everything that happened, and I hope he’s proud of this,” she explains through tears. Supt Clive Timmons, a raised eyebrow in human form, has the last word. He says that if Russia disagrees with this version of events, they can come to Britain and have their moment in court: “That’d be a good day out.”

There’s secrecy of a much more gentle variety in Inside the Freemasons (Sky1), which promises to offer “unprecedented access” to the inner workings of this ancient society. As part of their 300th birthday celebrations, they have been slowly opening up to journalists, and seem keen to burst any myths of a spooky secret cabal where men operate in the shadows and do something involving goats, a persistent misconception that one mason here finds especially hilarious. There is much to like about this film, which suggests that masonry is a benign forum in which men get to wear fancy suits and play in a world of secret codes and language, that to this uninitiated outsider sounds a lot like a form of cosplay, or the sort of thing you might find in the local historical reenactment society. It’s a shame the access didn’t stretch to the initiation ceremony that eager farmer James undertakes. My curiosity was piqued. I bet there’s a goat in there somewhere.