Ghost Alex is adjusting to life after death with werewolf Tom but vampire Hal is in hell. Not because he's chained up like an animal but because he's desperate to get his marigolds on the messy house.

But when Tom and Alex decide to set him free, Hal is racked by uncertainty. Can he control his bloodlust? Things get complicated with the return of Mr Rook, the shady figure whose government department protects the world from supernaturals.

Having been fired from the café, Hal and Tom find new employment at the Barry Grand Hotel, home to poisonous pensioner Captain Hatch. Unknown to our trio, his decrepit exterior hides an ancient evil that threatens not only their friendship but also the entire world.

Whenever we sit down to storyline a new season of Being Human we try to find a single headline – unofficial and usually never stated – that encapsulates the series arc. It's more of an aide memoire for us, something that articulates our ambition and intention.

Series One told the story of Mitchell, George, and Annie settling into their new home and the beginning of their (ultimately futile) attempt to ape humanity and live a normal mundane life. In my mind it was titled "There Goes The Neighbourhood".

In Series Two, our heroes were being hunted and manipulated by a shadowy group of religious zealots who had perverted the word of God to suit their own agendas, believing the eradication of supernatural creatures was a divine mission. I called it "God Loves, Man Kills".

Series Three dealt with the aftermath of the massacre Mitchell committed in Box Tunnel the previous year and his ultimate destruction at the hands of "The Wolf-Shaped Bullet", and Series Four detailed the vampire myth of "The War Child" – George and Nina's orphaned baby – and her part in destroying the vampire aristocracy.

When we started work on Series Five we gave it the title "The Greater Good".

Every year we try and up the ante. Blowing Mark Gatiss and a baby up at the end of Series Four was good, but we could do better. We wanted to tell a story that was even greater, even more labyrinthine and epic than anything we've done before. Slowly, a story about sacrifice and misplaced courage started to take shape. Our heroes would be driven to compromise their own fragile humanity to safeguard the rest of the world, and less-scrupulous characters would endanger the lives of others in pursuit of their own agendas.

Then, as other elements started to develop, we toyed with "He Will Rise" as our headline title. The merits of our main cast(s) have been well-documented, but we have also been lucky enough to have some of the finest actors the industry has to offer playing our Big Bads - Jason Watkins, Lyndsey Marshal, Donald Sumpter, Andrew Gower, Mark Gatiss - and this year we've been blessed to have Phil Davis join us, as the repulsive and cruel Captain Hatch, arguably our biggest villain ever. It's a performance of such delicious evil you'll be left with your jaw on your chest and your breakfast on your lap. He's joined by Steven Robertson as the mysterious Mr Rook, the sinister Man in Grey we met at the end of Series Four, and Colin Hoult as the deluded and volatile Crumb. A newly-recruited vampire, he describes himself as "the world's worst nightmare: the victim that suddenly gets super-powers".

But in the end we settled on "The Trinity", because this was the first entire series with our new cast. Thanks to the brilliance, enthusiasm, and sheer elemental talent of Damien Molony, Kate Bracken, and Michael Socha, it was their skill and passion that would stop me rocking and dribbling and draw me out from under my desk to actually write the scripts.

And so this series title belongs to them. The new heroes of Being Human. The new trinity.

Werewolf Tom spent his life on a mission: to kill vampires. He moved into Honolulu Heights after his beloved father-figure McNair was killed. Putting his nomadic lifestyle behind him and trying his best to quash his vampire-killing instincts, Tom soon became an emotional rock for his housemates. Learning to live a normal life has its challenges, but Tom's charming innocence and noble loyalty stand him in good stead. In Series Five, his ambition to be successful brings its own trials. Can he overcome them and beat the beast inside?

Hal has been a vampire for more than 500 years and the temptation for blood and carnage still rule his every waking thought. He came to live with fellow supernaturals Annie and Tom after his best friends crossed over to the afterlife. Though his standoffish nature and OCD tendencies didn't initially warm him to Tom, their affection for each other grew until they became confirmed buddies. Hal's addiction to blood proved too strong and he was – tangentially – responsible for Alex's death, marooning her in the afterlife as a ghost. At the end of Series Four, Tom and Alex strapped Hal to a chair in order to go through cold turkey… but will it prove wise to release him?

When we first met Alex in Series Four she was full of life. She didn't stay that way for long. Fun and flirtation with Hal soon turned sour when she was killed by vampire Cutler in retribution for Hal killing his wife 50 years ago. Hal then did the one thing he'll never hear the end of from Alex: he drank her blood. Marooned in the afterlife, Alex knows she's lucky to have two mates in Hal and Tom to face it with - she'd just never tell them that. Having died so young, she's certainly got plenty of unfinished business to attend to.

Captain Hatch has seen it all. At least he reckons he has. And whatever it is, he doesn't much like it. Stuck in a failing body, in a wheelchair, in a dismal seaside hotel, things aren't exactly sunny side up. Hatch embodies the worst in humanity – he's bitter, manipulative, obsequious, and cruel. Quite the catch. But those aren't even the worst things about the aged army captain. Beneath his decrepit exterior there lies within him a dark and toxic secret that could rattle your soul with fear.

An alien fleet stands poised to invade Federation space. The only vessel available to hold it back is the Liberator, commanded by Roj Blake and his crew.

As an intergalactic war breaks out, old enemies become allies, friends will become separated, and Blake will be forced to leave behind all that matters to him . . .