When he feels like losing control

Of course it couldn’t end well. In the original book “The Strange Case of Dr. Jekyll and Mr. Hyde” both fellows wound up dead, victim to the former’s inability to control the latter’s evil ways. So it’s not surprising that Saturday’s two-hour finale of BBC America’s modern-day “Jekyll” racks up a few casualties. It’s just too bad that the suspense and neck-prickling thrill of the previous episodes of what we still hope will be an ongoing series are among them.

Since its two-hour debut three weeks ago, Steven Moffat’s “Jekyll” has been a showcase for the extraordinary talent of James Nesbitt (“Ballykissangel,” “Bloody Sunday”), who plays both the staid Dr. Tom Jackman and his psycho counterpart, Mr. Hyde, without so much as a makeup change, save for the smoothing of his hair and the addition of black contact lenses. His Dr. Jackman is not so much repressed as haunted, withdrawing from his wife and family, like any good addict in denial, into a world of constant compartmentalization as his condition worsens.

His condition, of course, is the emergence of another personality, a libidinous, violent man of voracious habits and brilliant comedic timing. “Fact is Billy, I do have a nice side, but you just missed him,” he tells a thug he is about to beat to a pulp. That would be Mr. Hyde, who differs from Jackman not just psychologically but physically. By hiring a psychiatric nurse (played by Michelle Ryan, soon to be “The Bionic Woman”) and even enlisting the aid of an unlikely pair of private detectives, Jackman attempts to control Hyde and live some sort of normal life while trying to discover what the blazes is going on.

The tension between the two characters is riveting to watch, and Nesbitt’s performance proves that great acting can support all manner of nonsense, including a prolonged scene in which Hyde kills a lion with his bare hands. A secret society stalking Hyde for its own nefarious purposes provides the narrative drive, but that is much less fascinating than Nesbitt’s agile dance between the characters. Jackman’s increasing fear and bewilderment, partnered with Hyde’s jolly incursions into utter ruthlessness (though always taking care to hurt only bad guys) made for a perfect psychological partnership -- as a superman, Hyde gets to do what we all occasionally dream of doing and Jackman helpfully supplies the guilt and horror. The rest of the performers are no slouches either -- as the various women in Jackman’s life discover his secret, their reactions are far more interesting than the traditional gasp followed by fainting away. The thin line between terror and titillation is delicately toed.

All of which is well and good until that old secret society takes over, kidnapping everyone in sight -- Jackman, Hyde, the wife, the kids, the nurse, the detectives -- in an attempt to . . . what? I think they were trying to kill off Jackman so they could use Hyde somehow -- as a super soldier? A DNA farm to cure cancer? None of it made much sense and was far too reminiscent of the leaps of logic required of “Dr. Who” fans (not surprising since Moffat writes that as well).

Friend-turned-foe Peter (Denis Lawson), who worked for the society, keeps monologuing about the greater good and human evolution -- Hyde apparently represents the first example of real human evolution since bipedalism. But all of it takes way too long to explain, as does the origin of the whole Jekyll curse, and requires the use of both requisite stainless steel headquarters and old manor house with drippy dark basement. Not to spoil anything, but in the grand old tradition of English literature, a woman is at the root of all evil.

Sigh.

So much exposition and so little explanation, and all of it unnecessary. What we want to see is more of the fascinating marriage between Jekyll and Hyde, and if it is necessary to turn Hyde into some sort of sociopath for justice (Ã la Showtime’s “Dexter”) to continue the series, so be it. But no more conversations over brandy in the firelit study. Let’s just cut to the chase and let the adventure begin. Dramatically speaking, evil has always been far more interesting than evolution.

mary.mcnamara@latimes.com

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‘Jekyll’

Where: BBC America

When: 8 p.m. Saturday

Rating: TV-14 (may be unsuitable for children younger than 14)