First broadcast 19th July 1979.
Steel's drastic measures to save Sapphire and trap the pool of light prove effective, if extremely dangerous.
David McCallum ... Steel
Joanna Lumley ... Sapphire
Steven O'Shea ... Robert Jardine
Tamasin Bridge ... Helen Jardine
Val Pringle ... Lead
Ronald Goodale ... Countryman
Felicity Harrison ... Mother (voice)