The Rills are telepathic reptilian creatures with large scaly eyes and no vocal cords, considered to be hideous by some humanoid species. They come from an apparently pacifistic planet (presumably in Galaxy Four) with an ammonia-based atmosphere, which they require to survive, as oxygen is toxic to them.
A spacecraft carrying twelve Rills encountered a Drahvin spacecraft over a planet in Galaxy Four. Worried that the Drahvins would fire on them, the Rills stopped their ship and waited for four days. Eventually they turned the ship, at which point the Drahvins fired. The Rills returned fire, and both ships crashed on the planet. Eight of the crew were killed in the crash, leaving only four surviving Rills, who enclosed themselves in a room. They set up a machine to convert the planet's air into ammonia, which was pumped into the room to keep them alive.
The Rills' repairs were carried out by their dome-shaped machines, which Vicki calls Chumblies. The Chumblies have no sight, as such; they rely on touch, by feeling their way around strange objects, and hearing, although they can only hear things in front of them. They are armed with a rod-like weapon which fires both a paralyzing stun ray and a light ray which causes things to burst into flames. A flashing light crowns their dome, and they communicate in a high-pitched whistling sound. They also have mechanical arms with which they can pick up and carry things, and make repairs. The Rills can also transmit their thoughts to the Chumblies, who, given enough linguistic information, can then convert the thoughts into speech and broadcast them through speakers, giving the Rills a way to communicate verbally. The Chumblies are fairly impervious to attack, and are not hurt by flames, although when covered by a metal mesh, their control waves are blocked, rendering them incapacitated, at which point their dome collapses inward. They are, however, magnetized, so their top cannot be removed, meaning that not just anyone can tamper with their internal mechanisms.
For a pacifistic culture, the Rills nevertheless possess some formidable weaponry, including a black vial-shaped explosive set off through the wires attached to it, and non-lethal ammonia bombs.
After crashlanding on the planet, the Rills attempt to lend the Drahvins aid, but Maaga fires at them, so they leave her to her own devices and set up a drill to obtain enough power to launch their spaceship. When they learn that they have insufficient time to accomplish this, the Doctor offers to transfer power from his TARDIS. When this is successfully completed, the Rills take off from the planet and escape before it explodes.
