The TARDIS lands on the surface of the sea and the travellers use a rubber dinghy to get ashore, where they are promptly shot by tranquiliser darts and taken prisoner. They have arrived in a restricted area, a natural gas refinery, and there have been a series of problems in the pipes. One of the engineers insists that a rhythmic beating sound heard in the pipes must be a living creature. The Doctor discovers that the weed is repelled by high-pitched noises when Victoria's screaming drives it away and he uses a recording of the screaming to defeat the weed creature.