Complex sentence: They appeared in four films which ran between 1968 and 1973, and they became popular so quickly that it was decided to kill them off in the final short "Worn Tyres Kill" in order to prevent public demand for their reappearance.
Split into simpler sentences: They appeared in four films which ran between 1968 and 1973. The two became popular so quickly that it was decided to kill them off in the final short "Worn Tyres Kill" in order to prevent public demand for their reappearance.