Prior to their disappearances, they were a family of hat makers working for their company the Hightopp Hattery, which makes the best hats in Underland. During their lifetime, Tarrant lived with them until during the tragic Horunvendush Day event where the Jabberwocky terrorizes the festival and ruins it. As revealed in the sequel, it has been confirmed that the family did not die under the hands of the Jabberwocky, but rather captured by a group of Armored Cards summoned by the Red Queen.
At the present, the Hightopp Clan has been shrunk into the size of ants, where they are kept in the Red Queen's ant farm in the Queen's lair. When Alice finds the Hightopp Clan for Tarrant, he happily finds them after missing them for a long time. Later after order is restored back to normal in Underland, Tarrant sprinkles some Upelkuchen to restore them back to full size as Tarrant happily reunites with them.
The Hightopp Clan briefly appears in Tarrant Hightopp's tragic flashback, where they attend the Horunvendush Day festival to which the festival is ruined and the Jabberwocky arrives and ruins the festival.
Further details of what happened to Tarrant's family before their disappearance have been explained in the film, as it shows the origins of what happened to the Hightopps before their disappearance.
When Alice Kingsleigh returns to Underland, she finds out that the Hatter is in a terrible condition, which is explained that Tarrant is looking for his missing family just as the Hatter's hair begins fading from orange to pale white. This leads Alice to travel back in time to find out the evidence of what happened to the Hightopp Clan before they disappeared by entering Time's Castle where she talks to Time that his friend Tarrant is in the risk of dying to which Time refuses to believe at first. Using the Chronosphere to travel to the past events, Time notices of what Alice did to it as she could alter the past, the present, and the future in Underland, much to Alice looking for Tarrant's missing family.
Alice travels to the past Toomalie Day event where he encounters Tarrant's family attending the failed coronation of Iracebeth and Mirana to which the coronation was unsuccessful, much to the crowd's disappointment as Tarrant is scolded by his father for causing a disappointment to his family. Alice later travels to the past Fell Day event where she observes a past version of Zanik telling Tarrant to make a hat properly to which while studying, she gets the idea to know that the names of the Hightopp family are not in the Underlandians Deceased section in Time's Castle, but rather missing.
Alice returns to Underland again and uses the Chronosphere to travel to the Horunvendush Day event, which is the same flashback seen in the first film where the Jabberwocky terrorizes the festival and ruins it as the Hightopp Clan panics in fear of danger with the Town of Witzend on fire. Alice attempts to get close to them, but her path is blocked after the Jabberwocky spews fire on the path. All of a sudden, the Hightopp family is shown not to be dead as it is revealed that the Jabberwocky wasn't the one who killed them; instead, they are surrounded by a group of Armored Cards summoned by the Red Queen throughout this tragic disaster. The Cards then march towards the Hightopp Clan as the Hightopp family is captured and taken to the Red Queen's castle as prisoners just as the paper hat that Tarrant made falls on the ground, which is linked to the Red Queen's lair in the present.
At the present, Alice returns to Tarrant's home and finds out that the Hatter is not in his study desk, but rather in his bed as Tarrant is at the risk of death to which the Hatter's hair starts fading to pale white, much to Alice's friends mournfully leaving upon reacting to what has happened to the Hatter. Alice tells him that he must believe in her while telling him that his family is still alive just before the Hatter's hair turns completely pale white as Alice continues mourning over him. However, Tarrant finally wakes up after Alice told him that his family still alive as his hair turns from pale white to orange, just as he happily smiles at her much to Alice's joy of what she told him to which Tarrant realizes that his family is linked to the Red Queen as Tarrant is brought back to life thanks to Alice. Having saved Tarrant, Alice and her friends storm the Red Queen's lair to find Tarrant's missing family.
As Alice arrives at the Red Queen's lair, she finds her childhood ant farm upon seeing the outline of a hat on it, to which it is then known that the Hightopp Clan family members are shrunk into diminutive sizes as Tarrant happily reunites with them. The moment was disrupted when the Red Queen summons two Vegetable Soldiers to seize Alice and her friends. Tarrant manages to hold the ant farm to protect his family from danger while Alice plans to take Time back to his castle and bring the Chronosphere back in place before Underland perishes. Tarrant, holding the ant farm with his family trapped inside, travels alongside Alice back to the past Fell Day event to which the two manage to bring the Red Queen (who was turned into time rust) and the White Queen back to the present and put the Chronosphere back in place as the time rust starts to engulf everything as Alice, the Hatter (and his family), the White Queen, and the Red Queen (who was encrusted by the time rust) plan to return to the present. As the time rust occurs due to the Red Queen causing catastrophe, the rust begins to engulf everything in Underland, to which upon returning to Time's Castle, Tarrant gets caught in the time rust's path, engulfing him just as the rest of his family becomes encrusted as everything else in Underland becomes engulfed by time rust at this moment.
As the Chronosphere is put back in place, Tarrant and his family are restored back to life as everything is restored to normal. The Hightopp Clan gets released from the ant farm as the White Queen gives Tarrant some Upelkuchen as he sprinkles them on Tarrant's family as the members of the Hightopp Clan are restored back to their normal sizes. After that, Tarrant's family is happy to see their son back in joy as they reunite with him. Having found the Hightopp Clan and saved Tarrant's life, Alice leaves Underland and returns to the real world as a farewell.
- The only members of the Hightopp Clan whose names are mentioned in the sequel are Zanik and Tyva. The rest of them have their names revealed in the credits.