Thrown together through the luck-of-the-draw of the Torchwood recruitment process, this pair of newbie investigators could hardly be more ill-matched. Toshiko Sato is diligent, attentive, meticulous and aims to excel in everything she’s asked to do. Her fellow recruit Sebastian Vaughan is something else entirely.

Born into a family of wealth and privilege, Sebastian is a self-absorbed and selfish git, content to do the minimum he can get away with, evading all responsibility. Toshiko is a perfectionist, Sebastian is a slapdash chancer. Each finds the other almost impossible to work with, but while their personal and professional clashes stress Toshiko out, Sebastian sails through oblivious, only fleetingly irked by her insistence that they should do things properly.

This is the very promising set-up for the latest Torchwood monthly release The Vigil. Lou Morgan’s cleverly built script flips back and forth through the story’s timeline, showing the pair squabbling their way through different missions, depicting Sebastian’s unfortunate demise (an aspect of the story that Big Finish’s own promotion for the release immediately reveals), and exploring the repercussions of his death on those around him.

It’s a technique that ensures a much more engaging narrative than a simple linear pass through the same material would provide. In particular, there’s great drama in knowing that Sebastian’s end is coming whilst not being sure quite how he will fall victim to his own hubris and foolhardiness. His death is a tragedy that need not have happened on what should have been a routine entry-level mission.

Sebastian was fortunate to find himself doubled-up with Toshiko rather than someone far less patient with his maddening behaviour. In death, it’s those same impulses of duty and loyalty that lead Toshiko to honour Sebastian’s final wish to be returned to his family estate for burial in the Vaughan graveyard, joining inumerable dead ancestors interred in the same plot over the centuries. It’s only at the moment before his passing that Sebastian’s flippant demeanour fades, as he acknowledges the importance of familial tradition to his own identity.

Putting Tosh in a situation of dutiful high-stress is always a winner in the Torchwood stakes, and Naoko Mori puts in a great vocal performance that combines the right balance of exasperation and determination. Hugh Skinner is good value too as the insufferable posh boy, unable to see the seriousness of any predicament.

Sebastian’s mother Madeline, impeccably played by Lucy Robinson, is no less of a monster than her son. Ghastly, wretched and seemingly adrift amidst the trappings of fading privilege, her priorities reflect the oppressive weight of obligation that Sebastian was understandably keen to rebel against. It’s impossible not to feel empathy with Toshiko, having to deal, alone and unsupported, with two repugnant characters from the Vaughan clan, who could not be more repellant if they tried.

As events shift to the family estate, atmospheric sound design comes to the fore: especially when the drama moves outside for a heart-wrenching and ghoulish game of hide and seek. There are some genuinely spooky thrills as the chase unfolds: with voices lost in the fog, mixing with a mother’s plaintive lament, and Toshiko’s desperate efforts to keep everyone safe, ahead of a fitting final showdown.

Torchwood: The Vigil is available now on CD and download from Big Finish.