ROGUE ELEMENT!

The latest coerced agent of death for the menacing presence of The Black Guardian, new TARDIS occupant Turlough (Mark Strickson, giving an impressive performance in this intriguing Twentieth Anniversary story arc) joins the series with Mawdryn Undead and goes on to initiate an act of sabotage on the time vessel that will put the Fifth Doctor and his companions in a nightmare situation linked to the unusual realm of Terminus.