“The Lesson” is a writerly thriller that spends the better part of its first hour trying to convince you it’s not “Deathtrap Lite,” no matter how many times its author-antagonist insists “Good writers ‘borrow,’ great writers STEAL.”

And then the final 15 minutes kind of undo all that effort in the most contrived ways imaginable.

But it makes a grand showcase for the snobby, aloof villainy of Richard E. Grant, who devours his best role in decades like a starving man who breaks his fast with caviar and canapes.

Britain’s “most revered writer” needs an Ox-Bridge pedigreed tutor to prep his teen son for an entrance examination, and that’s what brings Liam Somers (Daryl McCormack, the sex-worker in “Good Luck to You, Leo Grande”) into the home of novelist J. M. Sinclair (Grant). Not that he meets his idol. Not right away, anyway.

Art dealer/curator wife Hélène (Julie Delpy) handles that hiring and moves our would-be writer/hero into the guest apartment in their posh country home. Liam will drill, coach and counsel nervous, touchy Bertie (Stephen McMillian) through this all-important admissions process.

The son of the testy, five-years-“blocked” novelist will be “reading” for a class place in literature, of all things, and father’s too busy and as we gather, entirely too judgmental and lacking in Oxford/Cambridge experience to manage that prep himself.

A tragedy hangs over this house, an older son Felix who died. Liam must navigate not just the whims of the upper class — some nights, he’s invited to eat with them at their classical-music-underscored dinners presided over by the imperious workaholic Sinclair, some nights he isn’t — but the “rules,” which Hélène lays out.

“We don’t talk about his work,” she says. “And we don’t talk about Felix.”

But as Liam has been led to understand that Sinclair has, from time to time, “used” members of the family and short-term employees as his muse, clerk, proofreader and “amanuensis,” the fellow who made sure to pack his own handwritten manuscript when he moved in has hope.

A framing “prologue” shows Liam as a writer sitting down for a public Q&A about his work, and a “prologue” guarantees an “epilogue,” unless the screenwriter’s forgetful or prone to cheat.

So we know Liam is going to get something out of this, no matter how long it takes for the imperious, rude (“Close the door on your way out.”) and dismissive Sinclair to invite the young man into his writing sanctum and routine, no matter how challenging teaching his stressed, striving kid turns out to be, no matter what he learns about the death of the other son and no matter how much of a MILF the lady of the house might seem to be.

That’s a flaw this screenplay never quite papers over. By the third act, narrative problem-solving is abandoned altogether for a goofily ghoulish finale.

But Grant shimmers and seethes as the “revered” yet resentfully-pressured author who demands fealty and brushes off praise and criticism the instant he sizes up the stature and status of his new “helper.”

McCormack is rather blandly inscrutable as Liam, a character given the sort of prodigious memory one only sees in movies or lazy sitcoms where there’s a need for someone with a British Library-sized capacity for quoting the Bard and every-bloody-body-else who ever published a poem or book in the Mother Tongue.

Good scenes establish Liam’s hard-won ability to “pass” in this world of letters, Lizst and Rachmaninoff. A question about his taste in music is treated like another quiz this working class (Dad was in “IT”) lad must pass with his “betters.”

Bad scenes have him recovering an entire “lost” manuscript, which he’s read once, from memory.

The middle act manipulations leave one hoping the efforts of British TV director Alice Troughton and screenwriter Alex MacKeith (“Chubby Funny”) will amount to something worthy of Grant’s grandiose performance. “Good writers borrow, great writers steal” and all that.

What do you say about a script that doesn’t manage either, despite luring an ace-cast with a few well-drawn characters and couple of sharp scenes that stand out from the melodrama surrounding them?

“Close the door on your way out?”

Rating: R — violence, suicide, sexual content, profanity

Cast: Richard E. Grant, Daryl McCormack, Stephen McMillian, Crispin Letts and Julie Delpy.

Credits: Directed by Alice Troughton, scripted by Alex MacKeith. A Bleecker St. release.

Running time: 1:43