The Rutan Empire once controlled most of Mutter's Spiral, but have suffered losses during their long war with the Sontarans. They originate from the planet Ruta Three, an icy planet. Rutans are amphibious, having evolved from the sea and adapted to land, and heat is painful to them. Physically, they resemble luminous green balloons, swathed in cottony swirls, with several long tentacles dangling from the bottom. They are slow-moving, but can scale walls easily. They seem to have a single eye, although it is not clearly visible on their bodies, and they do not perceive color. Their bright green glow is caused by the strong electrical field they generate - Rutans can drain off electricity, and they use their electrical field to deliver massive electrical shocks to their victims. They also seem to leave a residual electrical charge behind them. Rutans emit a bubbling sound, and crackle when angry. They can adapt their environment to the optimum thermal level, meaning that they cause a cold wave in their presence.
Rutan ships are crystalline. The motherships are large; scout ships are used for individual Rutans. They refer to themselves in the plural, possibly indicative of some kind of hive mind. Rutans are difficult to kill - they are not injured by knives, but heat (and possibly explosives, although the heat of the explosion may be the damaging element) can be fatal to them.
The Rutan scout whose ship crashed near Fang Rock was trained in new metamorphosis techniques. This organic restructuring ability, a kind of chameleon factor, enables it to change its shape at will, although it must study a lifepattern first. To this end, it carried away a human it had killed and did a post-mortem to familiarize itself with human anatomy. It also caused an unnaturally cold and quick fog to blanket and isolate the island. Furtive and cunning, it studied the land, calculating the physical strength of its enemies before infiltrating them with a human shape. It then set up a distress beacon to signal its mothership.
The scout had just discovered how strategically viable Earth would be when its ship crashed. The Rutan Army had been making strategic withdrawals to selected strongpoints, and intended to occupy Earth for its strong strategic position. The planet would serve as a launching pad for their final assault on the Sontarans. Leela was able to kill the Rutan scout with an explosive blast; when it died, it slowly fell apart into gelatinous bits. The mothership was eradicated with a laser blast.