The Brigadier calls the Doctor for help as he wants the Doctor to help him find out who has been attacking Scottish oil rigs. The Zygons homeworld is unknown but the Zygons were hiding under Loch Ness. They used a castle to get in and out of the village. They captured the villagers and they disguised themselves as them. They also built a Skarasen and made people think it was the Loch Ness Monster. The Zygons plan was to make Earth their new homeworld as their other one was destroyed by solar flares. Broton who was the Zygon's leader was to disguise himself as a very important man and head to London. The Doctor along with the Brigadier and Sarah Jane noticed that Harry wasn't himself and they found out that he was just a duplicate.
They did manage to get all the villagers out of the Zygon ship before the ship blew up in London thanks to the Doctor. The only Zygon survivor is Broton and he still disguises himself as the Duke of Forgill. Broton makes the Skarasen go through the Thames. Unfortuantly Broton's plan fails. Broton gets shot by the UNIT troops and the Skarasen homing device is destroyed. The Skarasen heads back to Loch Ness.
Apparently the Graske captured some Zygons but eventually they were released.
The Zygons have appeared in:
Season 13: Terror of the Zygons
2005 Christmas Special: Attack of the Graske
Mentioned in Series 5: The Pandorica Opens