The abiding question is “Why?,” not “How?,” and the search for an explanation for Nilsen’s actions is carried out by a pair of audience surrogates: Peter Jay (Daniel Mays of “Line of Duty”), the lead detective in the case, and the writer Brian Masters (Jason Watkins of “The Crown”), whose study of Nilsen, “Killing for Company,” is the screenplay’s source. They take turns, as interrogator and interviewer, sparring with the glib, smart, narcissistic Nilsen, trying to pull from him the names of his victims and a reason for their deaths.
It’s not a requirement, in that setup, that a drama definitively answer the question it poses. We accept, in the end, that there is no answer — Masters, in his book, cites the “essential unknowability” of the mind, and Tennant has called playing the role an effort to “illuminate the unilluminatable.”
But “Des” needs to give us something, and for all of its intelligence, superior craftsmanship and conscientious performances, it doesn’t really deliver. At the end of the show’s two and a quarter hours, Nilsen remains as opaque as he is when the police first knock on his door.
Which brings us back to Tennant, and the hair and glasses and cigs. His portrayal is technically flawless and, moment to moment, absorbing, but it feels completely exterior. This is partly, maybe largely, a function of the script, which in its determination not to be sensationalistic errs on the side of vagueness. (If the point is that Nilsen was just an empty shell, it’s not made in a way that I found very compelling or particularly chilling.)
But it also has to do with Tennant, who’s been wonderful playing showier villains in “Jennifer Jones” and in the British TV movie “Secret Smile” but doesn’t get under the skin of the more prosaic serial killer here. Tennant’s gift, from “Doctor Who” to Shakespeare, is for cerebral theatricality, not the nuanced banality of the Dennis Nilsen that “Des” presents. In keeping with the overall tenor of the production, Tennant keeps things under wraps. That may accurately reflect Nilsen, but for the sake of the drama you wish there had been a way for him to let it rip at least once.