Gary Oldman has found the role of a lifetime in Jackson Lamb, the hard-drinkinglodi 646, hygiene-challenged spy who commands a band of MI5 misfits in the Apple TV+ series “Slow Horses,” which presented the finale of its fourth season on Wednesday. The patina of filth that transforms Oldman into Lamb is child’s play to apply compared with the hourslong ordeal he endured for his star turn as Winston Churchill in “Darkest Hour.” The flagrantly flatulent Lamb represents a sly renunciation of the button-down George Smiley, the espionage establishment figure that Oldman portrayed so effectively in the movie adaptation of the John le Carré novel “Tinker Tailor Soldier Spy.”

The boozers, misfits and losers who inhabit the fictional MI5 purgatory known as Slough House spring from the mind of the British thriller writer Mick Herron, whose novels proved so hip that Mick Jagger agreed to provide the series theme song. Oldman has described Lamb as an instrument of affliction for failing spies who have been shipped off to his dismal outpost.

Speaking as Lamb of the spies he oversees, he said: “My main objective is to make their time at Slough House so bone-crushingly boring that they leave and remove themselves from the service.” This may be true on some days, but on others, he is the perverse, Rabelaisian father figure who urges his broken-winged charges toward their nobler selves.

It may seem sadistic when he pours himself a drink and also offers one to a longtime aide who is a recovering alcoholic. But as the critic and editor Thomas Jones has pointed out, this could be Lamb’s way of keeping his valued aide on the wagon by serving as a reminder of the catastrophic depravity she has refused to fall back into.

The paradox of the novels — and the series — is that the spymaster at their center is both essential and unknowable. Herron has said that he avoids writing from the inside of his beloved protagonist’s head because he does not want to break him. As the historian and New Yorker writer Jill Lepore has written, this omission stands out glaringly in novels that read like carousels on which the rider switches animals every time around.

“You’re in one person’s head, and then you’re in someone else’s, except, unnervingly, you’re hardly ever in Lamb’s,” Lepore wrote. “He’s a cipher, forever undercover.”

“Slow Horses” has thus far offered only intimations of the trauma that turned Lamb into a personal wrecklodi 646, while heightening his skill as a spy to extrasensory levels. The show’s producers may have started out thinking that it would be perfectly fine to leave the black box of Jackson Lamb closed. But now that the series is amassing a growing viewership and headed to its fifth season, pressure is mounting to let the viewers into his head.

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