He starts off as an agent of the Black Guardian in Mawdryn Undead who is trying to kill the Doctor but JNT decided he should be an ongoing companion and it's kind of hard to keep the narrative thread of a companion who is trying to assassinate the Doctor going over multiple stories without making Turlough look like a total retard. Strickson has said that not knowing what to do with him, the writers of the television series would often have the villains capture or lock him up, leading to Turlough ending up in "various states of bondage." This no doubt suited JNT just fine.
More JNT brilliance Edit
Apparently JNT originally wanted Mark Strickson to shave his head for the role of Turlough because he thought punk was hip with the kids (despite the fact it was 1983 and he was several years too late.) Fortunately Strickson refused.
Turbo eventually rejects the Black Guardian in favour of loyalty to the Doctor and travels with him until he fucks off home at the end of Planet of Fire.