Ashleigh Cummings and James Cosmo co-star in Alexander J. Farrell’s new horror movie, set around a British castle haunted by a dark family secret.

Of all the horror subgenres, the werewolf movie is perhaps the hardest to pull off.

Part of what makes the genre tough to crack is the creature itself, which never manages to scare us quite enough. This is in part due to special effects makeup and VFX that, more often than not, can look pretty ridiculous on screen. And it may also be because so many of us are dog lovers, which means watching a man-dog-thingy running around killing people doesn’t exactly terrify us.

It’s a smart move that lends an unsettling tone to this childhood horror story, which is told from the viewpoint of a little girl, Willow (Caoilinn Springall), growing up in a big ol’ haunted castle with a monster lurking close by.

That monster may very well be her father, Noah (Harington), whom we quickly begin to suspect has some major skin, hair and teeth problems each month when there’s a full moon. His wife, Imogen (Ashleigh Cummings), does her best to deal with them, offering him live pigs for supper and keeping Noah far enough away from home so as not to be too much of a danger to the family.

Farrell, who wrote the script with Greer Ellison, gets the most out of the constrained setup, following Willow as she slowly but surely learns the truth about her dad. Saddled with an oxygen tank due to a strange sickness, the girl is both fascinated and terrified by Noah, who can be fun-loving in one moment and totally threatening the next.

As its title indicates, The Beast Within is more psychological horror than gorefest, probing the emotional struggles of Willow, Imogen and Noah as they come to terms with their family crisis. Harington does a convincing job in that sense, playing a man who can be loving and seething — or is that teething? — at the same time. In one strong scene, he unveils his dark family history to Willow in a way that underlines Noah’s dangerous inner fragility, which is not something you see in every werewolf story.

Such murky details are what make the film intriguing but also a bit of a letdown — because the first rule of a good werewolf flick, or any horror flick for that matter, is to keep the audience on the edge of their seats, whereas Farrell mostly keeps us guessing.

He does have a knack for staging action in restrained locations, with the entire film set around the crumbling family chateau and misty neighboring forest. (The country and time period are unspecified, though it looks like it could be postwar England.) Both cinematographer Daniel Katz and production designer Russell De Rozario deserve kudos for getting lots of visual mileage out of what surely wasn’t a huge budget, using a colorful palette to make the creepy settings feel less claustrophobic.