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.

Rothwyn and Eldred, the couple living inside the space capsule, begin to realize they might be in serious danger when they cannot make contact with fellow study groups or with their superiors. The evil creature inside the capsule begins an attack by using time to transform the couple's infant into a grown man. As Sapphire and Steel deepen their investigation, Sapphire discovers that the mechanism which transported the capsule is no ordinary machine.

After Sapphire disappears while trying to contact the creature inside the capsule, Steel must figure out where she has gone and how to proceed with the investigation. Luckily, technology specialist Silver arrives to lend a hand with getting inside the capsule. Rothwyn and Eldred discover their child has gone missing and are at a loss for what to do next. Meanwhile, their adult-sized infant wanders the capsule under the control of the creature hidden inside the wall.

Once Silver concocts a way inside the capsule, he transports Sapphire and Steel into the experimental pod from the future. The couple are no where to be found but Sapphire, Steel, and Silver come face-to-face with the adult-sized infant. Sapphire deciphers that the overgrown baby is a changeling controlled by time. The changeling senses that the three time agents are a threat and the force controlling the changeling commands that he destroy them.

Sapphire and Steel subdue the changeling and find where the time source is hiding in the wall. Without their technician, however, they have no way to deconstruct the wall and defeat the creature hidden inside. Steel's decision to resort to drastic measures pays off when the couple reappear in the capsule and the changeling becomes an infant again. But the creature inside the wall still thrives and presents a larger threat than anyone could have imagined.

After Silver's return, the three time agents question the couple from the future about their origins and their knowledge about the machine that transported them through time. The couple reveals some horrifying information about the future that directly relates to the creature inside the wall. Now with an understanding of the creature's motivation, Sapphire and Steel must prevent the creature's escape and manipulate time in order to save all of humanity.