Agent Mortis was always seen as one of the Time Agency's most devoted servants. Little did the Time Agent's know that Mortis was always on Agent Liam's side. After sabotaging a court meeting, in the 39th century, Mortis and Liam ran away from the Time Agency and were then the Time Agency's "most wanted".
After collapsing after an attack from a Time Agency guard, Mortis witnessed Liam walk away into a blue box and travel away with the Doctor. Mortis then teleported to to Katrina Mills, Liam's girlfriend, and her baby. After explaining what happened, Mortis and Katrina agreed to save the baby by sending the baby away via his Vortex Manipulator. Mortis promised to find the baby, one day.
What happened between this event and Mortis' debut in Series One is currently unknown. It is during this period, however, that Mortis is trapped in the Mortis Code, which caused him to go insane.
The breach of the Mortis Code eventually cracked, in the 21st century, which allowed Mortis to simply out of the portal . Mortis, now insane, tried to hunt down an destroy each of Agent Liam's team before they eventually stopped him. He then joined them in the "Battle for the Blue Box" where, upon seeing Liam leave in the "blue box", Mortis was instantly reminded of the God-like Doctor figure and had realised that he had gone down a dark path and sought to correct himself.
However, Dan and Ben did not believe him and trapped him back in the Mortis Code where he remained until Liam released him, believing that Mortis had now turned on their side. It was then revealed that Dr. Robin Turner, a member of Liam's team, had a personal vendetta against Mortis since Mortis apparently killed Robin's old branch, in Cardiff, meaning Robin wanted to avenge their deaths.
After helping Liam's team defeat the Venators, Mortis was taken to the graveyard of Ben, Danny and Bob the Flower Alien. Mortis felt instant regret and seeked redemption. Liam welcomed Mortis to the team. After a mission with Dan in the Forest of Dean, in which he comes close to uncovering a dark mystery concerning Agent Liam and something called "The Circle", Mortis witnessed, and fought in, a war with Katrina Mills and the Zakatrons. He also witnessed the death of Dan the Cyborg.
The team was forced to split up until Judgement Day, although Mortis met up with, and later ran away from, Robin Turner, before proving his innocence.
After the Flower Alien God was destroyed and Liam disappeared from Dartchester, Mortis was left in charge with the instructions "follow in my footsteps".
It is now his turn to form a team to right wrongs and tackle alien crime.
"Get ready, because things are gonna get nasty."