Born in Scotland on November 23, 1945, Nilsen seldom saw his Norwegian father, who preferred strong drink and travel to the quiet life at home. Nilsen's parents were divorced when he was four years old, and his mother soon remarried. Joining the army in 1961, Nilsen remained in uniform until 1972. Upon discharge, he moved to London and became a policeman, moving on from there through a series of government jobs. A closet homosexual who holds the standing British record for serial murders, Nilsen would not kill for sex, like Dean Corll and John Gacy in the United States. Rather, his crimes appear to be the product of sheer loneliness, coupled with a morbid fascination for death. Keeping remains of his victims on hand for months at a time, Nilsen was (in the words of biographer Brian Masters) literally "killing for company."

Nilsen's loneliness was held at bay through 1976 and early 1977 by the presence of a live-in companion ten years his junior. While they apparently never had sex, the younger man provided Nilsen with friendship and someone to talk to, sharing the daily grind of cooking, housework, and so forth. Nilsen was stricken by his roommate's departure, in May 1977, and the pressures of a solitary life gradually mounted to the detonation point.

Nilsen's first victim, in December 1978, was an anonymous Irish youth whom he brought home and strangled with a necktie. Dennis later masturbated over the corpse, storing it beneath his floorboards until August 1979, when it was cremated in an outdoor bonfire. In November 1979, Nilsen tried to strangle Andrew Ho, a young Chinese, but Ho escaped and summoned the police. Confronted with a former colleague, officers accepted Nilsen's story of attempted robbery by Ho and let the matter drop. A few days later, on December 3, Nilsen strangled Canadian Kenneth Ockendon with an electric cord and dissected his body, flushing parts down the toilet while most of the butchered remains were stashed under his floor.

In May 1980, Nilsen murdered 19-year-old Martyn Duffey, hiding his corpse with the fragmentary Ockendon remains. That summer, 26-year-old Billy Sutherland joined the growing crowd, followed shortly by a victim who might have been Mexican or Filipino. "I can't remember the details," Nilsen said later."It's academic. I put him under the floorboards."

Memories were vague on the next five victims, their names unknown, identified only by some physical trait or quirk of behavior that stuck in Nilsen's mind. A young Irishman and a malnourished transient were brought home in swift succession, strangled to death in Nilsen's flat. Number eight was cut into three pieces, his remains hidden beneath the floor for two days before they were burned in another garden bonfire. Number nine was a young Scot, and his successor an unruly "Billy Sutherland type." Number eleven was a tough-talking skinhead, notable for the tattoo of a dotted line around his neck, with the instructions "Cut Here." Nilsen did, and the young man was incinerated on a bonfire during May of 1981.

In September of that year, Nilsen found epileptic Malcolm Barlow slumped against his garden wall and phoned for an ambulance. Barlow came back to see Nilsen the next day, on his release from the hospital, and it proved a fatal mistake. A month later, when Nilsen found new lodgings, he cleaned house with one last bonfire, the blaze leaving police with no evidence of twelve murders spanning almost three years.

A month after settling in his new apartment, on November 25, 1981, Nilsen attempted to strangle Paul Nobbs with a necktie. Nobbs survived the attack, which took place as he slept, but he made no report to police. The next victim, John Howlett, fought bitterly for his life, forcing Nilsen to drown him in the bathtub when strangulation proved ineffective. Howlett's remains were hacked up in the tub, then boiled down in a kettle before they were flushed through the drains.

In May 1982, Nilsen tried to drown Carl Stottor in his bathtub, changing his mind in midstream, persuading Stottor the assault had been intended to "revive" him after he had nearly suffocated in his sleeping bag. Next day, while walking in the woods, Nilsen crept up behind Stottor and clubbed him to the ground, but again Stottor survived, shrugging off the attack and filing no complaint until after Nilsen was jailed for multiple murder.

Number fourteen was alcoholic Graham Allen, killed and dissected in Nilsen's flat, portions of his body bagged and stored in the cupboard, while others were boiled and flushed down the toilet. A local "punk" named Stephen Sinclair was the last to die, murdered on February 1, 1983, portions of his body flushed down the toilet a week later.

It was finally too much for the plumbing, and Nilsen - like Joachim Kroll before him - was betrayed by his pipes. Tenants of Nilsen's apartment building summoned a plumber to clear out the lines, and his discovery of human flesh brought police to the scene. In custody, Nilsen freely confessed his crimes and was sentenced to life imprisonment. Asked about the motive for his murders, he replied, "Well, enjoying it is as good a reason as any."