King Julien has a brilliant idea: He'll tame a fossa and train her to keep peace in his kingdom. But the plan doesn't go quite as expected.
This is Mary Ann's first appearance.
The fossa are attacking the lemurs. Julien constructs a "food pyramid", to save only himself. However, thanks to Clover, the fossa was caputered in one of the traps she placed there. The foosa, seeing one of their kind trapped, run away. Clover asks Julien what to do with the captured foosa. Julien's idea is to "lemur-ize" the foosa, so she can return to her people and tell them to stop eating lemurs. Maurice and Clover eventually support the idea. So the "lemur-izing" begins.
Nightfall in the plane, Julien is frustrated his plan isn't working. Outside, Clover is guarding when she discovers Horst. Horst smells something odd, and proceeds to sniff Clover much to her discomfort. She sends him away. Julien asks Clover why there is so much noise, to which Clover replies that it was just Horst.
Julien continues talking to the foosa, knowing there is more inside of her, but the foosa's rope breaks and she starts chasing Julien. Julien runs to the throne room and grabs his keytar in a panic. The keytar starts to play music, at which the foosa stops attacking Julien. Julien learns that the foosa enjoys the music and starts to play music for her. The foosa licks Julien as appreciation and tells him that her name is Mary Ann. Julien, learning her name, tells her that it is time for the "lemur-izing" to begin.
The next morning, Julien, with the help of his friends, teach Mary Ann how to dance, read books, and practice social etiquette. After some relaxing in the hot tub, Julien decides that it is time for Mary Ann to meet the kingdom. In the background, Horst is looking displeased with the events at hand.
At the gathering, King Julien introduces Mary Ann to the lemurs, but the lemurs' immediate reaction is panic at the sight of the foosa. Mary Ann gains the lemurs' trust after she performs a song and dance routine with Julien and Ted. After Mary Ann greets the lemurs, she promises to help the kingdom any way she can.
While walking, Maurice congratulates Julien for his idea working, and even Julien is surprised his idea worked. They meet with Mary Ann who is teaching lemurs about rebellion and regicide. Maurice is worried, since regicide means the killing of the king and tells Julien. Mary Ann is about to ask the lemurs if Julien is a good king, before Maurice stops her pulls her over to have a word with Julien. Julien calls Mary Ann his favorite foosa, but asks her to not teach crowds about killing kings. In the bushes, Horst even more suspicious about the events.
In the Throne Room the next morning, King Julien is woken by Mary Ann, who has made a list of how to make the kingdom better, and runs off to set up the suggestion box. Clover appears, distrustful of Mary Ann, stating that "King Julien VIII had a suggestion box, until a rebellion broke and they buried him in it, alive". King Julien looks at Mary Ann's list and agrees with some of them until it suggests that he get rid of different types of dance parties. Julien admits that he made a monster and agrees with Clover that they have to get rid of her. If it all goes horribly wrong, they decide Mort is the scapegoat.
At a gathering, Mary Ann tells the lemurs how a suggestion box works. King Julien and Maurice watch from above, hoping Clover will fix yet another of Julien's messes. Clover drops cooking spices on Mary Ann, making her hungry. She starts rounding up the lemurs to eat them. King Julien and Maurice run from Mary Ann, toward Clover. Clover, thinking she's got Mary Ann, unleashes her trap, which backfires as it unfortunately captures her, King Julien, and Maurice. Apparently, Mary Ann changed out her traps days ago.
In the hot tub, being cooked by Mary Ann, King Julien questions Clover's plan of cooking spices. Clover says phase one worked so well that she didn't really plan a phase two. King Julien says to Mary Ann that this was all Mort's fault. Mort, showing no guilt, is eaten first. Mary Ann is about to eat Mort, but gets interrupted by Horst calling out her name. Mary Ann stops, recognizing Horst. Julien asks Horst if he knows this foosa, to which Horst responds "Yes. This foosa... is my wife!" This shocks everyone. Mary Ann explains to Horst that the last time she mauled him, she thought she killed him, which led her to run away. Horst says he survived and everything grew back, but his heart is broken from what Mary Ann turned into. Horst asks Mary Ann to be with him again and offered her a flowery necklace. Mary Ann puts out the fire under the hot tub and leaves with Horst, kissing him.
The next day, King Julien, Maurice, and Clover decide to check up on Mary Ann and Horst and wish them a good morning, but Julien steps on Horst's back. Maurice asks what happened to Horst. Horst replies that Mary Ann mauled and left him again. Horst says that communication wasn't the problem between the two, but his loud snoring, that drove her away and back to her own kind. Julien, even though it didn't work out as planned, is glad there is a foosa out there who is able to lead a more fulfilling, happy, and peaceful life.
In Foosa Kingdom, Mary Ann is now queen of the foosa, and promises to teach and lead them to rule Madagascar, and the world. The foosa being not that smart, are confused at Mary Ann's words. Mary Ann says that this will be a lot harder than she thought.