If James Wan’s supernatural horror had ended abruptly after the first hour, it would probably be the creepiest thriller to haunt the big screen since the original Paranormal Activity.

Suspense cranks up relentlessly until a large knot of tension pins us to our seats, unable to look away from the unfolding terror through trembling fingers.

Unfortunately, screenwriter Leigh Whannell, co-creator of the bloodthirsty Saw films, engineers a hare-brained second act that completely alters the mood.

All of that pent-up tension dissipates as preposterous twist follows ludicrous turn and we find ourselves holding our breaths not through fear but to stifle giggles of incredulity.

Insidious is truly a film of two halves, the latter section nodding and winking affectionately to Poltergeist as the central characters journey between the corporeal and ethereal realms to exorcise a malevolent spirit from the family home.

Josh Lambert (Patrick Wilson) and his wife Renai (Rose Byrne) move into a new house with sons Dalton (Ty Simpkins) and Foster (Andrew Astor).

Late one night, Dalton hears a strange sound in the attic and foolishly goes to investigate.

He glimpses a shadow and lets out a blood-curdling scream.

The next morning, Foster tries to wake his brother but the boy has slipped into a coma.

Three months later, Renai is taking care of Dalton at home while Josh works to keep a roof over their heads.

The wife senses something terribly wrong in the home and Foster adds to Renai’s discomfort when he whimpers about an intruder: ‘I don’t like it when he walks around at night.’ So Josh’s mother Lorraine (Barbara Hershey) invites her supernaturally gifted friend Elise Rainier (Lin Shaye) to survey the property, flanked by assistants Specs (Leigh Whannell) and Tucker (Angus Sampson).

‘There’s something about this house, it’s like a sickness,’ discloses Elise, who fears that Josh and his clan might not be mentally strong enough to face the dangers that lie ahead.

Insidious initially holds us in a vice-like grip, following the Paranormal Activity template by escalating the threat to the family from creaking doors and strange shadows to full-blown physical violence.

Wilson is a likable hero, who conveniently plays dumb when Elise remarks, ‘You’ve grown a lot since I last saw you,’ and he gels convincingly with Byrne.

Hershey attacks her thankless supporting role with gusto until Shaye enters the field of psychological battle and reveals the film’s ultimate design through expository dialogue.

Wan and Whannell pay homage to the horror series which started their careers with a chalk drawing of their iconic puppet on a blackboard in one scene, but here they eschew gore and entrails in favour of frayed nerves.

For that at least, we should be thankful.

• Release Date: Friday 29 April 2011
• Certificate: 15
• Runtime: 102mins