Sapphire and Steel receive a puzzling assignment at an apartment complex where time is poised to attack. However, their inspection of every room in the complex reveals nothing suspicious. Even the abandoned top floor reveals no indications of trouble. It is only after investigating the roof that Sapphire and Steel discover an invisible time-capsule from 1500 years in the future. A couple and their new baby are living inside the capsule as an experiment, but they are in grave danger because of a horrible figure hidden inside the capsule with them.