It's a nicely fleshed out role for Rebecca Hall (usually a supporting actress), who offsets the inevitable screaming with an overall cool and calculating approach. Ironically, this is what makes her vulnerable.
Florence looks very much in charge, initially, brusquely dispelling the smoke and mirrors at a bogus séance. The scene also captures the period brilliantly with its mahogany post-industrial age mechanisms and pointless frippery. Of course, Florence is a buttoned-down type of lady, except that she doesn't hold back with her opinions, even publishing a book to state her case.
This brings her to the attention of Mr Mallory (Dominic West) a WWI veteran and history master at the aforementioned school, an imposing institution that's all at once grand and claustrophobic. From the moment she sets foot there, the sense of so much cold, airy space runs chills up the spine.
West plays it a touch frosty as well, but the attraction between the two is just as evident. Both characters are repressed in different ways and debuting film director Nick Murphy teases out their secrets with an expert touch. At one point he inverts a scene from Psycho, putting Florence at the peep hole to see what Mallory is made of beneath all the starch. As it happens, he bears a terrible war wound, but so does Florence in a less obvious way.
Confiding in a lonely boy (Isaac Hempstead Wright), she reveals how the war has impacted on her own life. Ostensibly, she is there to investigate a suspicious death and offer assurance to terrified pupils that it's a case of natural causes rather than ghostly disturbance. In fact, she has her own demons to exorcise.
As hidden passions rise to the surface, so too does the terror, with Florence forced to question her entire belief system. But even at the height of fear, Hall shows a steely backbone; determined to know the truth. Again, Murphy inspires dread with acres of echoing space and shifting shadows. He's not above using a few cheap shocks as well, but thankfully, he's sparing with flashes of the ghost who we sense is close by. Imelda Staunton hangs around a lot too as the housekeeper, but does a good job of keeping us guessing about her allegiances.
What a shame that Murphy loses the plot in the final stretch, recycling a worn-out twist and overcomplicating the backstory to try and justify some cheating. After such a deliciously tense build-up and subtle nurturing of the relationships, it all goes up in a puff of smoke. Up until then, you'll be holding your breath.