The Doomed Planet is the 5th Produced episode and the 5th Broadcast episode of the series Fireball XL5.
A flying saucer leads the crew of XL5 to Membrono, a planet doomed to be destroyed by another planet which has come out of its orbit. An old man sent the saucer to lure Steve Zodiac to Membrono where through Robert, he asks him to use Fireball's powerful missiles to destroy the drifting planet before it destroys Membrono and in turn his own world, which is governed by it. Hearing this, Steve is able to help the old man and save his peaceful world.
- Venus has had Zoony for three months and has taught him to say, "Welcome home."