Synopsis: The Third Doctor is on his way back to Earth (and UNIT) after an unspecified journey in space. As he nears Earth, the TARDIS' instruments—and his own hearing—are confounded by a piercing noise. Tracing the source of the noise, he comes to a military testing range in coastal Scotland.
There, he finds that the British military have been testing a new sonic weapon. While it successfully scuttles an offshore boat, it is continuously transmitting sound waves at higher frequencies that can't be heard by humans. Unaware of the damage they're doing, the military's new project is sending worldwide panic throughout the animal kingdom. If not stopped, it threatens to upset the natural order of animal life on the planet.
After the typical difficulties in establishing his identity to the military—it takes an awfully long time for the commander of the squad in charge of the new weapon to put a call through to the Brigadier—the Doctor finally manages to convince them that the project must be scuttled.
Most particularly, the controlling satellite in orbit around Earth must be destroyed. At first the Americans have a go at shooting the thing down, but their missile misses because the satellite's transmissions screw up the guidance system. This puts the destruction of the satellite down to the Doctor. Using his sonic screwdriver to cut through the noise, he guides the TARDIS into a close parking orbit with the satellite, and destroys it (presumably by, in some way, manipulating its own signal against it).