He serves as Charlie's spy to steal inventions and ideas for him. He went undercover as head of supplies and procurement of Energy Research Lab. None of his colleagues thought very highly of him.
When he attempted to attend a high-level meeting regarding the power source of Jake the alien cat's spaceship, he was thrown out, told that if they needed pencils they'd call him.
He worked as an agent for the greedy and powerful Charles Olympus. Whether he was always one of Olympus' henchmen and was planted at ERL, or if he worked there before he started spying on the Army for Olympus out of greed for money or possibly resentment for how his colleagues treated him, is unknown. He worked under the codename "Jellyfish," and even Olympus didn't think very highly of him, as when he tried to contact his boss about the meeting with General Stilton, Olympus initially didn't remember who he was.
Determined to figure out what the military was up to, he tried and failed to break into the base where Jake's ship was being held. However, his ERL colleague, Frank Wilson (not noticing he was there), broke in with the help of Jake and his magic collar. Stallwood followed them and witnessed all the magical feats the cat's collar could grant whoever possessed it, including the power of levitation and flight.
Later, he managed to get evidence of it using a portable video camera, and, armed with this, he went to Olympus' lair and showed it to him and his henchmen Mr. Smith and Omar. Despite the footage being upside-down down to Stallwood having to film hanging from a second-floor balcony, it was enough to fire Olympus' imagination and promise to make Stallwood a very rich man if they could get the collar.
Stallwood then went with Olympus and his men to Wilson's home, where they abducted Wilson's girlfriend Liz and her cat Lucy Belle, along with Wilson's neighbor Link. Olympus released Link so he could deliver their demands to Wilson - Jake's collar in exchange for Liz. Then they went to an airfield and waited inside Olympus' Gazelle helicopter.
Stallwood expressed an extremely naive world outlook where his far more ruthless boss was concerned. When Wilson and Link arrived with Jake, Stallwood believed Olympus would release Liz was promised, however Olympus revealed he planned to kill her to protect his employee, so she wouldn't reveal Stallwood's involvement to the authorities. Stallwood was horrified.
When the Army and police arrived, chasing Wilson, Link and Jake, Smith, possibly Olympus' most trusted aide, thought it was a trap, and on this hunch Olympus ordered Omar to fly them to safety. The Gazelle flew away, but Wilson and Jake gave chase in a beat-up biplane Jake made fly using his collar. After an intense aerial pursuit, Smith started shooting at the plane. Stallwood grabbed a flare pistol and tried to join him, but accidentally fired the flare off inside the helicopter. The controls were damaged, and the Gazelle would now only fly straight ahead until its fuel ran out.
Smith and Omar donned parachutes and bailed out, leaving the helicopter without pilots. As Olympus put his one chute on to join him, Stallwood realized he hadn't been given one, and begged Olympus not to leave him. Olympus tried to jump, but Stallwood grabbed his lapels and wouldn't let go, and the two men fell out of the open door together. In freefall, Olympus' chute opened successfully, and Stallwood still clung desperately to him.
They ended up in a tree together, dangling over a chasm, and Stallwood attempted to cut Olympus free with a pocket knife, but Olympus, realizing they hung over a cliff and would fall to their deaths if he cut the straps, threw the knife away and called him an idiot.