In the drama, viewers witnessed Nilsen, played by David Tennant, confess to killing 15 men in London from 1978 to 1983.
However, only six bodies were ever recovered – which is why he was charged with six counts of murder and two of attempted murder.
Nilsen’s first victim was Stephen Dean Holmes, who was only 14 when he met the killer in a pub called the Cricklewood Arms on December 30, 1978.
The Irish teenager had attended a pop concert in Willesden, but hadn’t managed to buy alcohol at the pub.
The Scottish killer invited him back to his flat at Melrose Avenue where they continued to drink until he fell asleep.
Nilsen strangled the boy with a neck tie until he was unconscious, before drowning him in a bucket of water.
Nilsen kept his body in the flat, more specifically under the floorboards, for several months, and even slept beside it.
It was decades after Stephen went missing that Nilsen finally admitted to him being the first victim.
Nilsen, a civil servant who had previously worked in the Army and police, was never charged with the office because at the time of his trial, Stephen couldn’t be identified.
In 2006 the killer sent a letter from his prison cell where he shared shocking details of the tragic murder.
He explained that he was afraid the boy would leave him, and he decided to stop that from happening.
Writing about the first murder, he admitted: “I had started down the avenue of death and possession of a new kind of flatmate.”
Nilsen disposed of the body on a bonfire in August 1979.