Allan Grimson joined the Royal Navy in 1978, rising to the rank of Petty Officer. He served aboard HMS Illustrious and later became a firefighting instructor at HMNB Portsmouth. During his tenure, Grimson reportedly targeted trainees and cadets, selecting those he found attractive to dominate and potentially harm.

Grimson's first known victim was 18-year-old Nicholas Wright, a Royal Navy sailor he met during a firefighting course in November 1997. On December 12, 1997, after a night out in Portsmouth, Grimson invited Wright to his flat. When Wright rejected Grimson's advances, Grimson attacked him, first punching, then bludgeoning him with a baseball bat, and finally slitting his throat. He later disposed of Wright's body near Cheriton, Hampshire.

Exactly a year later, on December 12, 1998, Grimson murdered 20-year-old Sion Jenkins, a former Royal Navy sailor working as a barman in Portsmouth. After meeting at a nightclub, they returned to Grimson's flat, where Grimson forced Jenkins into non-consensual acts before killing him. Jenkins' body was found near West Tisted, Hampshire.

In December 1999, following a tip-off, Grimson was arrested and confessed to both murders, describing the thrill he experienced as "better than an orgasm." He was convicted on March 27, 2001, and sentenced to life imprisonment with a minimum term of 22 years.

Grimson is the prime suspect in the disappearance of Simon Parkes, an 18-year-old Royal Navy sailor who vanished on December 12, 1986, in Gibraltar while HMS Illustrious was docked there. Despite extensive searches, Parkes' body has never been found. The coincidence of the date and Grimson's presence have led authorities to suspect his involvement.

Authorities believe Grimson may be responsible for up to 20 additional murders, given the pattern of his known crimes and his tendency to kill on December 12. However, no further charges have been brought against him.

In 2019, Grimson was moved to an open prison in preparation for potential release, sparking public concern. As of October 2022, a parole hearing was scheduled, but the outcome remains undisclosed. ​