Conversation so far:
What is Encounters with the Third Doctor?
The Third Doctor (Jon Pertwee) convinces the Master to stop this plan at the last minute.
Who encounters the Third Doctor?
CANNOTANSWER.
Who is The Master?
The Master, played by Roger Delgado.
Is there anything interesting about this article?
In The Sea Devils (1972), the Master is shown to be imprisoned on an island prison off the coast of England.

Question: Why is he imprisoned?
Self-contained rewrite: Why is the Master imprisoned in Encounters with the Third Doctor?