Peter Davison Doctor Who Actor Matthew Waterhouse Adric Actor Sarah Sutton Nyssa Actor Janet Fielding Tegan Jovanka Actor Anthony Ainley The Master , The Portreeve Actor Dallas Cavell Head of Security Actor Frank Wylie Ruther Actor Michael Sheard Mergrave Actor Derek Waring Shardovan Actor Souska John Child Actor Fiona Cumming Director Christopher H. Bidmead Writer John Nathan-Turner Producer