'I can't get my head around it'. On witnessing 'Love Without Limits', the first episode of his new documentary series, chances are Louis Theroux is not the only one feeling confused. This British nosey-parker par excellence dips his ever-inquisitive mind into the impossibly tangled web of polyamorous couples (and throuples) in the US. 'Only in America' many would think. 'Only in Portland' would retort much of America, as Oregon's largest city exercises its extremely open-minded muscles.

So, we meet Nick and Bob, who are both in a relationship with Amanda, with all three residing under the same roof. We see how they cosy up at night while Nick reveals how he and Bob divvy up their moments of intimacy with Amanda. Meanwhile Jerry believes that having a wife (Heidi) and two lovers is an arrangement that would make most people jealous of his domestic set-up. The look on his face when Heidi beds down for the night with her own poly partner tells a different story.

The signage in Portland does a lot of the heavy lifting for Theroux's underlying concerns. In one almost too-perfect shot, a piece of street graffiti blasts out the city's motto of 'Keep Portland Weird!', directly above a rudimentary parking sign and another message reading 'Heartbreak Dead Ahead'. There's barely-hidden pain, loneliness and dissatisfaction bubbling above the surface of many of these lives. As has been his way over the course of three decades of documentary-making, Theroux sensitively probes his subjects, asking some tough questions while never being remotely judgmental.

As downbeat and morally challenging as that episode is, 'Choosing Death' ups the ante with Theroux meeting those taking advantage of the laws in seven US states which allow the terminally ill to take their own lives with a deadly cocktail of prescription medicine. The deathbed scene of 74-year-old Gus, a former musician and adventurer stricken with pancreatic cancer, surrounded by family and friends, his audible death throes almost drowning out the proclamations of love to a father, husband and pal is surely one of the toughest televisual moments of this or any other year.

Episodes watched: Two of three

Louis Theroux's Altered States starts on BBC Two, Sun 4 Nov, 10pm.