The Secret of Happiness is the one-hundred and fifty-fifth issue of the Winx Club Comic Series.
The Winx goes to Australia. A friend of Roxy, who manages the reserve, needs help. There, they realized a couple of Quokka is missing its baby. The fairies will have to find out who is involved in the kidnapping and learn the secret of true happiness.
In the Animal Rescue Park, the Winx are fixing up the place and caring for the animals as per Roxy's request. Roxy enters the park and congratulates the Winx on their work but says she still requires their help. Roxy explains that she just called her friend, Justin, from Australia, who manages a refuge for endangered species near Perth, taking care of animals before putting them into their natural habitat. Justin said that one of the animals of the refuge has disappeared mysteriously and it is very serious. Roxy says she cannot leave the center, so Stella offers for her and the Winx to go to Australia.
On the airplane to Australia as Musa and Bloom discuss their impending mission, Aisha, Tecna, and Stella discuss where in Australia to visit, mentioning the Outback, a desert in the heart of Australia. At the airport in Australia, Musa spots Justin in the crowd as he waits for them. Justin welcomes the Winx to Australia and introduces them to Pepper, his pet Dingo. Aisha researches Dingoes in her book as Stella struggles with her shopping bags and concludes Dingoes are dogs native to Asia but widespread in Australia. Outside, Justin loads all of Stella's luggage into his car and everyone then makes off to the refuge.
On the way, Justin tells Bloom and Flora that he knows the Winx's magic is useful for solving mysteries and Stella gets acquainted with Pepper. Aisha takes in the sites of Australia with her binoculars. At the Outback Animal Resort, Justin gives the Winx a tour of some of the animals in the sanctuary he has treated, introducing the to Hop, a kangaroo he found in the Outback with a wounded paw, Lucky, a platypus born a week ago, and Wally, a hedgehog with the snout of an anteater. He then brings the Winx to the reason for which they are in Australia, Berry and Coco, a couple of Quokka who he should be setting free tomorrow. Aisha researches Quokka and finds that they are Australian marsupials know as the happiest animals in the world, but Tecna says the present state of the Quokka contradict that statement as they do not look happy.
Justin says this is because their baby, Boing, went missing a few days ago and that not even Pepper, who has a phenomenal nose, could pick up their scent. Justin says that if he can get all the Quokka free there may be a chance to save the Outback. Just then, Arabella rolls up to the sanctuary in her car and tells Justin that she heard about the Quokka's disappearance. She says that she hopes he finds the Quokka or else it will be a big dishonor for the animal refuge to have lost such an exemplary specimen, but Justin angrily tells her to go away as she is not welcome.
Justin tells that Winx that Arabella is a rich person who lives nearby. He and Pepper think she has something to do with Boing's disappearance, but they have no proof. He says that on several different occasions that she tried to get Justin to sell her Boing, but he refused. Bloom tells Justin not to worry as the Winx will help him in the search for Boing, much to Justin's gratefulness.
Just then, Aisha is alarmed as a young girl runs by and drops her bracelet, who Justin reveals to be Arabella's daughter, Milly. Justin says that he knows where he and the Winx must start their search: the Villa Rivers, Arabella's home. Upon arrival, Justin knocks on the door to find the villa's butler saying that Arabella is not home, but Justin threatens to call the police if they are not let in immediately. Arabella comes out with luggage telling the butler to leave it to her, but Justin quickly demands that Arabella return the baby Quokka.
Arabella asks Justin to leave, but Justin says he also wants to return Milly's bracelet. Arabella demands it back, but Justin says they have business to take care of first. Inside, Arabella admits to taking the baby Quokka, but invites the Winx and Justin into Milly's room to see the reason why she stole it. Inside, they see Milly playing with Boing on the floor. Arabella explains that a few weeks ago, she took Milly to Justin's sanctuary and felt a spark between Boing and Milly, and since Justin refused to sell it to her Arabella decided to take it.
Justin explains that the Quokka is an endangered species, and no matter how much happiness he brings, he is not meant to be a domestic animal. Bloom defends Arabella by saying that she did it for her daughter, however, if Milly feels unhappy then Arabella should spend some more time with her. Having a realization, Arabella goes to talk to Milly, but Milly runs out of the house saying that Boing is her only fiend, unwilling to give him up. Arabella and the others run after Milly.
At the shore, Milly places Boing in a boat and tells him not to be sad because they will run away together. Just then, Boing and his parents back at the refuge begin to resonate together as they escape their enclosure. Milly and Boing head out onto rocky seas as the Winx, Justin, and Arabella plead for them to come back. The storm intensifies and Milly begins to cry for help. Aisha holds her head trying to communicate with the water but she says it does not answer her as they are of magical origin - they are trying to bring Boing back home.
Bloom directs everyone's attention to the boat and says that the Quokka have activated a magical power. Milly sees Boing's reunion with his parents and sees him smile for the first time. The energy resonates with the Winx and Flora says that the Quokka are giving them a new transformation to save them. Just then, the Winx transform into Magic of Joy fairies and work together to stabilize the waters and bring the boat back to shore. The Winx reunite Milly and her mother.
Arabella apologizes to Milly for neglecting her and promises that they will spend more time together in the future. Flora then tells Milly that even Boing has a mom and a dad, trying to get Milly to see the importance of returning Boing. Milly agrees, even though she will miss him so much. Justin tells Milly that she can visit Boing at the refuge, much to Milly's excitement as she and Boing can still be friends forever. At the animal refuge, Boing is reunited with his family as the Winx, Justin, Arabella, and Milly look at them in happiness.