Dalek Sec along with the other memebers of the Cult of Skaro who were Daleks Caan, Thay and Jast were created by the Emperor to think of new ways to keep the Dalek race alive.
The Cult of Skaro were hiding in a void ship inside Torchwood 1. There plan was the take over the world. After the ghosts revealed that they were Cybermen, Dalek Sec and the others came out of the sphere and into Torchwood. Dalek Sec then talked to the Cyber Leader after Dalek Thay killed two Cybermen. Dalek Sec kept saying lines against the Cyber Leader saying Daleks were supreme and that the Cybermen were weak. Dalek Sec then killed Dr Rajesh Singh to get information and kept Rose and Mickey alive.
The Doctor then met with Dalek Sec and saw that they had a Genesis Ark which was a prison ship. Then with the help of some Cybermen, the Doctor fought against Sec and the Cult along with the preachers. Dalek Sec and the Daleks killed the Cybermen in the room. The Cult of Skaro headed to the aliens technology area and Dalek Sec opened the roof mechanism and headed into the sky with the Genesis Ark. Dalek Sec then opened the Ark and release millions of Daleks who were imprisoned and let them loose on the world. Then the battle began and the Cybermen fought against the Daleks. The Doctor only had one option and that was to suck the Daleks and Cybermen into the Void. It worked and all of the Daleks and Cybermen were sucked into the Void. Except from Dalek Sec used emergency temporal shift and teleported away with Caan, Thay and Jast.Then Dalek Sec returned in 1914 Manhatten underneath the Empire State Building. Dalek Sec stole humans and turned the dumb ones into Pig Slaves who stole more humans. The smarter humans would be used in the final experiment. Dalek Sec summonded Mr Diagnorus and Sec's mutant sucked Diagorus inside of him. Dalek Sec was the final experiment. Later on the Doctor and Martha were taken to the laboratory and they saw Dalek Sec who looked like he was dying. He was then given an antidote to stabalise him. Then when the time was right Dalek Sec shut down and then opened up. Everybody even the Daleks were shocked to see what emerged out of him. Dalek Sec was now a human dalek who was now called Dalek Sec Hybrid.
The Doctor asked him what he felt but Sec said anger and pain. The Doctor then escpaed the Daleks along with the other humans. Sec sent Daleks and Pig Slaves after him. But the Cult of Skaro were concerned about Dalek Sec questioning if he was still a Dalek or if he was turning into a human. At Hooverville the Daleks attacked and killed Solomon. Dalek Sec told them to keep the Doctor alive and he told the Doctor that he said that the death of Solomon was wrong. The Doctor was shocked and Sec showed him the new Dalek race which were stolen humans who were part human and part Dalek. Then when Sec said that the Daleks should be extinct the Cult were shocked and even when Sec asked the Doctor to help the Daleks! After the Doctor's help though Sec found out that the Daleks were putting 100% Dalek DNA into the new Dalek Humans. Sec was then chained up to the wall and Dalek Caan took command. When Thay and Jast entered the theatre they had Sec chained up to them. When they were going to kill the Doctor, Sec got in the was and was killed. The Doctor was shocked as Sec could have helped the Daleks. Afterwards the Human Daleks killed Thay and Jast. Then Caan killed the new Dalek Humans. Caan was the only survivor from the Cult of Skaro and the Daleks. So the Doctor thought. Caan then did what Sec did at the Battle of Canary Wharf, emergency temporal shift.
Dalek Sec has appeared in:
Series 2: Army of Ghosts and Doomsday
Series 3: Daleks in Manhatten and Evolution of the Daleks