A rare example of a species that sought to invade Earth without using military might, the Koggnossenti attacked Earth from another dimension, apparently spending months in hiding before mounting their assault.
While the Koggnossenti’s full history is unknown, they were clearly advanced technologically, with their primary weapon in their invasion of Earth being an ultrasonic signal that could disrupt the mental faculties of the targeted race, with prolonged exposure causing the subject’s intellect to rapidly diminish. As depicted, a particularly extreme example saw Jill Meadows, a CEO of a major electronics company, become unable to understand how to use her own product after a few hours’ exposure to the signal, reaching a point where she couldn’t even comprehend how a pencil worked less than an hour after The Doctor met her when she was already a victim of the transmission.
When the Tenth Doctor and Donna Noble found themselves in the middle of the Koggnossenti invasion while visiting Eartha couple of years in Donna’s future, they soon realised that an invasion was underway when they witnessed people begin to panic in the face of ordinary technology. Fortunately, Donna and their new friend Bex were somewhat more resistant to the signal as they hadn’t been exposed to the Koggnossenti’s earlier transmissions – Donna had naturally been with The Doctor before arriving in this time and Bex had just returned to England from a gap year travelling – but this only delayed the degeneration of their intellects rather than stopping it completely, with Bex finding herself scared by a simple escalator and Donna having trouble recalling a plan she came up with a couple of hours after they realised what was happening. While the signal targeted humans, it was even able to affect The Doctor’s mental faculties, leaving him unable to properly use the sonic screwdriver and nearly having a panic attack when he returned to the TARDIS due to the scale of its complexity.
Despite his initial ‘shock’ at the TARDIS interior leaving him almost unable to speak, The Doctor was able to concentrate once outside the ship to ensure that his language abilities would remain intact, albeit at the cost of limiting his ability to come up with more sophisticated plans. When Donna and Bex discovered a group of Koggnossenti in the London Underground, Bex was captured when she panicked in the presence of an escalator, but Donna was saved by train driver Kevin Jones; as was later revealed, the Koggnossenti signal only affected 99.2% of Earth’s human population, and Kevin was part of the 0.8% who were totally immune to it.
With Kevin as a ‘secret weapon’, The Doctor and Donna confronted the Koggnossenti directly, allowing themselves to be captured while Kevin set up a train to ‘charge’ at one of the power lines the Koggnossenti were tapping into to power their equipment. With the signal temporarily disabled as the Koggnossenti tried to reactivate their equipment, this return to his normal self allowed the once-again-intelligent Doctor to confirm how the signal affected the human brain and then subtly reprogram the machine so that it would affect the Koggnossenti instead. Having realised what The Doctor had done, the Koggnossenti attempted to flee before the signal could completely take away their intellects, but by the time they activated their cross-dimensional ship, they had forgotten how to set its dematerialisation program properly, with the result that their ship exploded, scattering their remains across various sub-dimensions.