A Reactive Automated Trail Seeker, also known as RATS, was a rudimentary artificial intelligence which was semi-alive and with a shared consciousness throughout units. They were scavengers, often found near war zones to make off with valuable technology. They were programmed to fit in with their surroundings, and had the ability to metamorphose their hands into energy weapons which turned all they touched to antimatter. They spoke in metallic voices, and were programmed to be clearly understood, which led to them listing synonyms for the final word in their sentences.
In London in 1939 a RATS attacked Hetty Warner, believing she had a Gallifreyan "augur's stone", fallout of the Last Great Time War, which they had been tracking in order to further develop their artificial intelligence. However, the Ninth Doctor saved her by attracting its attention with his sonic screwdriver and running off. The next day, a group of eight RATS surrounded the Doctor and Warner, but abandoned them in order to track their primary target- the stone, which the Doctor had activated remotely with his screwdriver.
As they trapped Winston Churchill and his associates in Admiralty House, Warner accidentally accessed the power of the stone and learned that the Doctor had been trying to save the RATS from overloading their shared consciousness with the stone's power, as it would overload their minds. Nonetheless, they secured the stone despite the Doctor's pleas; they briefly achieved their goal of superior intelligence before overloading. (AUDIO: The Oncoming Storm)