In its pure misjudged ickiness, bad-acting ropiness, and its quirksy, smirksy passive-aggressive tweeness, this insidiously terrible film could hardly get any more skin-crawling. And its periodic attempts at lightening the tone with comedy are more chillingly humourless than anything I have seen in a long while.

It is a sucrose feelgood/feelsad drama or revenge weepie – always chivvying you to smile through your tears – about an adorably smart and caring 11-year-old kid called Henry, played by Jaeden Lieberher with a teeny little savant I-see-dead-people voice, a performance style pioneered almost 20 years ago by Haley Joel Osment in The Sixth Sense.

With his kid brother Peter (Jacob Tremblay), Henry is being brought up by his stressed but cheerful single mom Susan: a deeply unrelaxed and unconvincing performance from Naomi Watts. Henry is pretty much a prodigy level genius who, when not absolutely crushing his homework, has secretly made his mom a fortune by dealing in stocks from a payphone near their house. He has also apparently designed and built a huge and elaborate treehouse/den in their backyard, with sweet Heath Robinson inventions in it, the sort of thing that would take a grownup movie set-designer months to build. The film is always ordering you to believe in how joyful and life-affirming Henry is.

But Henry has a problem: he is (correctly) convinced that the teenage girl next door, Christina (Maddie Ziegler), is being abused by her creepy stepdad, Glenn (Dean Norris); he is a local cop, totally untouchable, and his brother is head of child services. Christina is tired, withdrawn, hollow-eyed, with all the symptoms. So when Henry heartbreakingly gets a fatal brain tumour, he leaves behind detailed instructions in a book for how to stop Glenn and take a terrible revenge on him, along with a taped pep talk for Susan to listen to on her headphones, hilariously anticipating her problems and missteps in real time, almost as if the seraphically wise Henry is actually talking to her from beyond the grave.

This excruciating premise is pretty much borrowed from Cecelia Ahern’s romantic drama PS I Love You (2007), about a young widow opening letters from her dead husband telling her how to pick herself up and start her life all over again. But it could have been acceptable, perhaps even brilliant, if it had had the courage of its apparent convictions and followed through with its revenge threat. But the film insists on pouring syrup over everything and trying to have its cake and eat it; revenge without actually doing anything nasty.

Combining improbable adventure caper, sickly bereavement drama and prurient child-abuse nightmare would present a tough tonal problem for any film-maker. It is certainly too tough for everyone involved here, and the film insidiously tries to use comedy as the general solvent, as if the periodic unfunny lines establish its general good faith and make sure we’re not too sad or too scared.

How very sad to see Sarah Silverman roped into this as Susan’s best buddy Sheila – they are waitresses at the local diner, a job which is there to showcase their indomitable good humour. Silverman’s gift for acid black humour, of course, goes to waste. And Watts, whose talent is unquestionable, is once again cast as the bog-standard female lead: a performance without flavour, without presence, without style. But it’s down to the writing. Can she please have a word with her agent?