First Mentioned In:
Last of the Cybermen
The Isos Network
Isos II was a planet in the Isos System. It was home to a colony of Isons and giant slugs. Cybermen attacked Isos II, turning it into a conversion planet which they used for 5 years. During this time, they worked towards invading Earth through International Electromatics.
The Second Doctor, Jamie McCrimmon, and Zoe Heriot came to Isos II following the IE invasion and destroyed the Cyberman presence on the planet. (The Isos Network)