Beast from Within is the eighteenth episode of the first season of the animated series, Martin Mystery. It was originally released on April 27, 2004.
A mysterious power outage at the center causes a creature to escape and now M.O.M. is extremely worried. The creature that escaped was Gastromo, the first creature M.O.M. ever caught when she was an agent. When the agents go and search for him, he attaches to one of the agents and takes control and is off to the Center to get his revenge on M.O.M..
A slug creature named Gastromo escapes from the center. It infects Martin first with him going through strange side effects such as him eating raw meat. Gastromo inside of Martin captures Jenni and cocoons her. Then Diana figures things out but then Java and M.O.M. get infected as well. All three of the them (Martin, Java, and M.O.M.) go after Diana.
She gets to the Center (M.O.M.'s office) and Diana figures that since Gastromo has been eating so much iron it has made him magnetic and she is right because she hovers an electromagnet over them and it attaches them to it. She then drops them into a big fish tank in the office and everything infecting them is expelled. Gastromo jumps out of Martin and tries to escape but is eaten by a fish. All of them talk about what happened and then Martin and Jenni go on a date only for Martin's clumsiness to ruin it.
- Toby (rabbit/cat hybrid)
- The Center's Agents
- Principal Pebbleton
- Siren (in a cell)
- Werewolf (in a cell)
- Ms. Doray (in a cell)
- Mutated Martin, Java, and M.O.M. (formerly)
- Slime Scan