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.

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

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

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).

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 likeable hero, who 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, but here they eschew gore and entrails in favour of frayed nerves. For that at least, we should be thankful.