The Doctor, Jamie and Victoria are taken prisoner by the staff of a gas refinery, who have lost contact with their drilling rigs and experienced mysterious drops in pressure.

As the situation worsens at the refinery, Chief Robson refuses to halt the gas flow. Elsewhere, the weed extends its influence and Mrs Harris receives sinister visitors.

Despite the mounting evidence, Chief Robson refuses to acknowledge that matters have escalated beyond his ability to control them.

Investigating the impeller shaft, Van Lutyens is overwhelmed by the weed creature. A call comes from one of the off-shore rigs - the weed creatures are attacking and taking over!

As the weed breaks into the Pipeline Room at the refinery, the deranged Chief Robson kidnaps Victoria in a helicopter and heads for the Control Rig.

The Weed has complete control of Chief Robson and threatens to conquer Earth. The Doctor searches for a means to defeat the creature and believes the answer may be Victoria...