1x11 - Sapphire & Steel

With Sapphire as a medium, Tully and Steel are able to communicate with the ghost of a woman who was once the soldier's schoolteacher. As they prod the spirit with questions, she reveals that the force helping and controlling the ghosts is a terrible, living darkness. She also gives them the name of the soldier and the details of his untimely death. Satisfied with the information, Tully and Steel finish up the séance but an enveloping darkness creeps through the room and threatens to devour them all.