"Space Monster" is the eleventh episode of the first season of Fireball XL5. It originally aired in the United Kingdom on ATV on January 6, 1963.
The episode opens with two members of the XL2 Crew stranded in a cave on a foreign planet, and one of them is terribly ill. Suddenly, the cave starts rumbling. Al, the sick crew member, says that they're not going to make it out of the cave alive, and suddenly, a monster shows up outside the cave entrance. The crew tries to keep quiet, but the monster peers in to try and find them. Meanwhile, at Space City, Commander Zero is trying to establish communication with the crew of Fireball XL2, but to no avail. Commander Zero orders Lieutenant 90 to call in the captain of Fireball XL5, Steve Zodiac, and his crew and have them go rescue the crew of XL2.
Steve gets the call, and the XL5 crew hops to it. Turns out, the Fireball XL2 sent out a distress signal, and the signal is coming from the planet Monotay. Within one hour, Steve, Venus, and Zoonie are in the rocket with the others and blast off to Monotay. Meanwhile, the monster has the crew of XL2 cornered and trapped like mice. Meanwhile, the crew of the XL5 goes to bed, while Steve stays up. Back at Space City, Commander Zero and Lieutenant 90 try to contact Fireball XL5, thinking they should have reached their destination sooner.
Soon, Fireball radios in, explaining that they're almost at Monotay, and the reason they were quiet for a while is because a while back, they ran through a heavy curtain of meteorites. And so, the Fireball XL5 crew continues with their mission. Soon, the crew lands Fireball Jr. on Monotay. Meanwhile, at Space City, Commander Zero contacts Professor Matic and asks him if he got any message from Steve and Venus, but he has not gotten anything since they landed. So, Commander Zero tells him to keep in touch. With that, Matic contacts Fireball Jr and asks them if they found anything, but Steve and Venus have not found anything yet.
Soon, the radar scanner on Fireball Jr goes off, meaning that they must have found XL2. Suddenly, something hits Fireball Jr, and they are forced to land. Steve tries to contact Professor Matic, but no response, causing him to think that the crash must have knocked out the radio gear. Soon, the two leave Fireball Jr, and find the crashed remains of Fireball XL2. Suddenly, Steve hears what he thinks is an electrical storm, so they both duck into a cave, the very same cave that the crew of Fireball XL2 is in. Once one of the crew mates explains what happened to them, Steve and Venus spot the monster.
After Venus checks on Al, Steve notices that the monster is gone. Actually, the monster has gone to investigate Fireball Jr. Venus tries to contact Professor Matic, but the portable radio is dead. Back at Space City, Matic contacts Commander Zero, and tells them the situation, and adds that he's going after them. Meanwhile, the team formulates a plan to help Al and get rid of the monster. A few hours later, Fireball XL5 prepares to land, finds the cave, and lands outside it.
However, the monster spots Matic and chases after him. The Professor enters the cave, just in time. Some time later, Steve heads to Fireball Jr to get some guns powerful enough to kill the monster, while one of the XL2 crew covers him. However, Steve ends up breaking his leg, and putting him at the mercy of the monster. But just before the monster can make a meal out of him, Zoonie saves him by sounding exactly like the monster. Soon after, they all make their way off of Monotay and back to Earth.