He and Bloo are among the first people to encounter a Fusion. After a Terrafuser lands in Foster's Home, he and Bloo stepped outside to investigate. Fusion Spawns emerged from the space rock and headed towards the house. Mac suggested that they might be new imaginary friends. After Bloo confronted the Fusion Spawns and told them to leave, the Fusion Spawns began to overrun the pair in a huge swarm. Dexter soon arrived with Ben (Four Arms) to protect Foster's Home. After the defeat of the Fusion Spawn, Bloo began to gloat, claiming it was he who saved the day, and if it hadn't been for him, Dexter wouldn't have been able to stop the Fusions. Mac shrugged him off and thanked Dexter forsaving them. The group soon headed to Sector V to help Numbuh Five, Numbuh Two, Blossom, and Bubbles defend Sector V. After a quick save, they headed back to Dexlabs to discuss the situation. After Dexter and Professor Utonium discussed methods of defeating the threat, Dexter proposed a plan to blast the main Terrafuser with his new Electro-Pulse Cannon, while a group of other people would attack the other two Terrafusers. He requested that the heroes team up to complete the mission. With everyone unsure about the idea, Bloo decided that he wouldn't help at all, forcing Mac to make him stay and convince the other heroes to cooperate in the effort. Dexter soon revealed that the two most important parts to make his EPC work were at Mandark Industries and Mojo's Volcano. Dexter and Utonium proposed that there will be one team to acquire one part and another team to acquire the other. Mac, Numbuh Five, Dee Dee, and Bloo went to sneak the needed part out of Mandark Industries. After fighting their way through a wave of Mandroids, swinging across a lake of alligators, and sliding down a pipe, they encountered Mandark. They told him how important the part was; his longing for Dee Dee convinced him to cooperate, on only one condition: that he might come along with them to observe.
The two teams regrouped on board Dexter's helicarrier. Dexter refused to work with Mandark and ordered him off the helicarrier, but Dee Dee convinced the two to work together. The helicarrier was soon attacked by flying Fusion Spawns. Mac and the others held off the Fusion monsters while Dexter finished up the EPC. Unfortunately, Mandark was distracted by his infatuation with Dee Dee and made a serious error in programing the EPC, causing it to jam at a crucial moment. Dexter was able to fix the error, and fired the cannon at the main Terrafuser, but not before it shot forth a signal that brought Planet Fusion to Earth.
In the Past, Mac seems to have run off to find Coco and ended up in the KND Jungle Outpost upon Mount Blackhead. Mac is helpful to the heroes, but seems to want to stay on the sidelines during the fight with Fuse, helping by using his imagination to make a false Coco to distract Fuse. Mac's support to the Imaginary Friends against Fuse shows his true loyalty and care to Foster's.
When Fuse tries to kidnap Coco, Mac takes it upon himself the task of protecting her from the Fusions in the area. He first sends the hero to defeat the Creeper Kings for their Leafy Claws. Once they are defeated, the Hero returns the Leafy Claws to Mac; Mac explains that the claws look like Coco's hair, thus demonstrating his plan to create a Coco decoy.
Nano MissionTrying to get Mac clear of Coco, Boarbills steal Mac's backpack. Mac assigns the hero to recover his stolen property from the Fusion monsters in Really Twisted Forest. The hero retrieves Mac's backpack out by the edge of the Forgotten Falls borderline, only for Numbuh Four to reveal (via NanoCom) that there is a gooby trap inside and the hero is forced to get rid of it. After the backpack has been thrown into the waterfall, Numbuh Four informs Mac and the hero that there is a Fusion in the area. Mac, drawling on Coco's words about a Fusion Mac in Forgotten Falls, suspects that this is the Fusion in question. The hero scouts out Fusion Mac near an island at the bottom of the waterfall. The hero enters the Fusion lair, but is rushed upon by Striding Lampreys. After defeating nearly all the Fusion monsters, the hero stumbles upon Fusion Mac. The hero hastily defeats Fusion Mac and creates a Mac Nano.
On April 1, 2009 FusionFall released seven new world missions from mission givers: Mac, Ed, Numbuh Four, and Numbuh Three.
- What happened to Mac's bag after this is unknown, despite the fact that he is still wearing it when the player visits him.
- He and his friends are among of the few people whose ages have remained the same from their original cartoon counterparts, as others have been aged up; the others like Mac include Ben, Rex, Finn, and Jake.
- His Nano, during the period of the original thirty-six Nano set, was the only one whose Fusion Lair was located outside of an infected zone. That has since changed, as most new Fusion Lairs for Nanos are located outside of an infected zone (e.g., Eduardo, Chowder, Rath, Ice King, AmpFibian, etc.)