Jason Priestley plays Coker, one of the survivors of the solar storm who helps Bill to take on the Triffids, the carnivorous plants whose relentless march is endangering the future of the planet.
The Canadian actor, 39, has adored John Wyndham's novel since childhood.
"I read it in high school and immediately related to it," beams Jason. "So when the producers contacted me about this, I knew exactly what they were talking about and was instantly intrigued."
The actor, who as a young man broke millions of teenage (mainly) female hearts round the world as Brandon Walsh in Beverly Hills 90210, also felt an immediate connection with the character of Coker.
"He is a very altruistic individual," reflects Jason.
"After the solar storm, when so many people get blinded, he is doing his best to help those afflicted. He's trying to take care of all these newly-blinded people.
"At that moment, Coker also seizes the chance to advance his own political beliefs, which are somewhat left-leaning. You could actually come out and says he's a bit of a communist. He believes he can start a new society where the very few sighted people look after the blind."
At first, Coker's fervent beliefs cause friction with Bill, who is leading the battle against the marauding Triffids.
"Coker is very dogmatic about things," asserts Jason, who has also starred in My Name Is Earl, Medium and Colditz.
"Coker's a tough customer who's willing to lay it all on the line for his cause. Initially, he clashes with Bill, but soon they are forced into forming an alliance – albeit a tenuous one! Coker's initial aim is to help as many people as possible, but it soon becomes a simple struggle for survival."
So does Jason think that The Day Of The Triffids has something to say to us today?
"Absolutely," he declares, without hesitation.
"This story is so enduring – you can apply it to any era. The political landscape of 1951 may no longer be relevant, but the abiding themes of the book still have great resonance. This story says that, if it is left unregulated, greed will bring us all down. As we were making this series, the world was disintegrating as the global economy collapsed. How's that for topical?"
Jason reveals that he really enjoyed working in this country.
"I love filming in Britain," he beams. "On The Day Of the Triffids, we shot in so many iconic locations around London. We were even allowed to film at the Gherkin, which doesn't happen very often. That was so awesome!"
He continues: "This is the third project I've shot in the UK, and I always have a great time here. Whenever the opportunity comes up to work here, I take it. I feel a real connection with you guys. I'm from Canada and my wife is from St Albans, so I feel a great kinship with the Brits."
Finally, the actor reckons that audiences will be scared out of their wits by the enemy confronting the last remaining humans in this drama.
"These Triffids are seriously mean! I hope viewers will be genuinely terrified by them – I know I was!"