Isis was the cat that accompanied supervisor Gary Seven from his secret base to Earth on a 20th century mission.
Isis was involved in the 1968 sabotage and detonation of a nuclear bomb in the atmosphere just 104 miles above the surface of central Eurasia. This near-calamity made the rival governments cease, for a time at least, the deployment of nuclear weapons in orbit, and rethink their old strategy of "balance of power".
Though apparently Seven's pet, Isis was more: she communicated with him through a rudimentary telepathy, and understood his spoken words. She accompanied him wherever he went and watched out for him. Isis was also able to take Human form, though which of the two might be her original form (if either) remains a mystery. The one thing that never changed was the collar around her neck.