Space Pen is the 15th produced episode and the 10th Broadcast episode of the series Fireball XL5.
Two crooks manage to steel isotopes from Space City. The crew of XL5 go after them to the planet Conva, but find trouble there. Steve Zodiac tricks the crooks into thinking that he and his crew are also villains and have stolen XL5. But the leaders of the gang are Mr and Mrs Space Spy, who recognize the crew of XL5 and order their death in a water chamber. The controller of the planet, General Shan, is able to rescue the crew and capture Mr and Mrs Space spy.
- The S.S. Thor is shot down by XL5 and completely destroyed here, so this episode come after Convict In Space, in which the Thor also appears.