The folly of Mel Gibson backing a former assistant with writing credits on “Apocalypto” and creating of TV’s “Boss” for Gibson’s most recent passion project, “The Professor and the Madman” is obvious, even to Gibson. They ended up in court over it.

But there was enough in this multi-handed script to pair up Gibson and Sean Penn as the title characters in a story of madness and a herculean endeavor that provides each a chance at “redemption.”

And a Who’s Who of British character acting royalty climbed aboard — Natalie Dormer, Jennifer Ehle and Eddie Marsan, Steve Coogan and Stephen Dillane among them — to tell the story of the daunting task that was writing the first edition of the Oxford English Dictionary.

The film is misshapen, fictionalized in seemingly unnecessary ways. But the cast ensure that it is never less than watchable, and even its problematic script has hints of the poetic glory that Gibson saw in it, the pathos of this unlikely story tangled up in one of history’s great undertakings.

And clumsy missteps aside, it’s still well worth watching.

Gibson stars as James Murray, a self-described Scots “autodidact” whose many interests and obsessively studied fields made him a Renaissance man of Victorian Britain. But the Renaissance was hundreds of years earlier, and when we meet him, the snobbery of academic aristocracy is curtailing the lexicographer, linguist and philologist’s efforts to take over a project no one has been up to the challenge of completing — an Oxford commissioned dictionary of the English language. He would “fix all spellings,” give histories of words, the common, the odd and the archaic. And he’d chisel, in stone “the tongue at its purest peak.”

The snobbiest academic (Anthony Andrews) and the would-be publisher (Laurence Fox) aren’t having this accented Scot do this sacred work. The school teacher and headmaster who has had academic papers published on language and dialect doesn’t have the degrees and credentials to edit such a tome.

“Autodidact” means “self-taught,” after all.

But Murray has a champion on the committee in charge, Frederick James Furnivall (Steve Coogan). He carries the day, and implores Murray to “let us begin with ‘aardvark’ and not finish until we read ‘zymurgy.'”

But with a tiny staff (Ioan Gruffudd, Jeremy Irvine) and a titanic task, they cannot finish without “crowdsourcing,” a word that hadn’t been invented yet. They needed volunteers to read the whole of literature in English, find words, write them down, cite the context (book, page, etc.) and quote their usage. Pleas for such help turn their purpose-built office, his “scriptorium,” into what looks like a scene from “A Beautiful Mind” — thousands of pre-Post-It note paper-slips covering tables and decorating walls, just to get a definitive history and origins on “approved” or “art.”

That’s how “The Madman” connects with Murray. Former U.S. Army surgeon Dr. William Chester Minor (Penn) had moved to London, fleeing his demons. Sure that he’s being chased by a Civil War tormentor (the film takes place from the early 1870s to 1908), he chases an innocent stranger and kills him.

Dr. Minor was acquitted of the murder and sentenced to an asylum, where a sympathetic “alienist” (pre-psychotherapy) in charge (Stephen Dillane) and a skeptical guard (Eddie Marsan) glimpse just enough of the man of “breeding” and the educated surgeon to indulge Minor on what becomes his new purpose, helping Murray in his quest.

The cleverest visual touch of this lovely, Dublin-shot drama is giving Minor his own “Beautiful Mind” room of obsessive, organized slips of paper tracking words through their published history, just like Murray, whom he’s never met.

Voracious reader Minor’s mountains of mailed-notes tell Murray that “God has sent us a savior.” It took him years to realize that this learned, well-read American was mentally ill. But as you can’t have a film where the stars don’t have lovely scenes together, that is rather glossed over here.

When Gibson’s Murray enthuses, “We are linked now — ‘consanguineous!'” — that glossing is excused, as are pertinent facts like Murray’s longtime friendship with Alexander Graham Bell and Minor’s earlier work helping produce the 1864 edition of the American “Webster’s Unabridged Dictionary.”

But when you dabble in a deep connection between the sometimes sentient/occasionally-raving Minor and the widow of the man he killed, you’ve lapsed into melodrama, and clumsily taken the picture’s “redemption” into places that hijack a cluttered story that already has Civil War flashbacks, Oxford villains and Murray’s family life (the regal Jennifer Ehle takes on a Scots accent as his wife), imperiled by this labor.

Gibson’s “Braveheart” polished Scots burr suits the bearded Murray to a T, a man who loves language, with a prodigious memory and years of work that teach him that many a gap in a word’s etymology can be filled by consulting the epic poem “Paradise Lost.”

“The language took a crucial turn with Milton!”

Penn is well-suited to Minor as well, a screaming, raving lunatic capable of calm, lucid protests that “I am NOT insane sir” in court even as he starts to come to grips with his crime and the burden that adds to a troubled mind.

It’s easy to see what the stars, acclaimed but with public histories that range from problematic to nearly-indefensible, connected with in a story about “what we pray for, what we whisper to our children…redemption.“

But Farhad Safinia, who co-wrote the script and took an assumed name as director (never a good sign) loses the thread and scatters the poetry of words at play — the professor and the madman bandy tricky entries (“Kumquat.” “Oblong.”) back and forth in their visits.

The clutter of it all includes the anachronistic treatments of the apparently-fictional “therapist” Dillane plays, a man who “diagnoses” Minor via phlebotomy (discredited three decades before the film’s timeline) and treats him with lobotomy (“invented” 40 years later).

As “unfilmmable” as a movie about men lost in words, attempting to write a dictionary might seem, there is a better picture in this subject, based on journalist/history buff Simon Winchester’s best selling book. Limiting its scope, beefing up the connection between the “consanguineous” correspondents, their letters and their meetings, giving the two men competing agendas (acceptance by academia vs “redemption”) rather than shoehorning both of them into one and losing the “love story” would have been a start.

Yes, that sounds like a play. But by the end of “The Professor and the Madman,” that’s the best fate you could wish for it.

MPA Rating: unrated, violence, profanity

Cast: Sean Penn, Mel Gibson, Natalie Dormer, Jennifer Ehle, Steve Coogan, Ioan Gruffud,
Stephen Dillane, Laurence Fox, Anthony Andrews and Eddie Marsan.

Credits: Directed by Farhad Safinia, script by John Boorman, Todd Komarnicki and Farhad Safinia, based on the Simon Winchester book. A Voltage production, Vertical Entertainment release on Netflix.

Running time: 2:03