The 30-year-old admits he is delighted to be returning to English football after a year on loan at Portsmouth, and said: "My agent arranged for me to come here because I need to play.
"I terminated my contract in Greece and I wanted to come back to England. Plymouth is a good team and the manager wanted me here."
Douala previously spent time training with Charlton, who are rock-bottom of the Championship and look doomed to relegation.
The striker added: "I think Charlton are now finished. They are probably going down and their chairman did not want to contract a player if they are going down."
Coca Cola Ch'ship Relegation 08/09 Win Outright: Plymouth 15/8