It should become clear within a minute of pressing play on HBO Max’s “Harry Potter” 20th Anniversary special (“Return to Hogwarts”) whether you’re in or out for the nearly 2-hour special to come. The dreamy opening evokes the canny, warm charm of a Christmas commercial, to the familiar tune of John Williams’ iconic score (with an extra helping of jingle bells for good measure). The camera homes in on actors Emma Watson (who played Hermione Granger), Matthew Lewis (Neville Longbottom), and Robbie Coltrane (Hagrid) enjoying a crisp winter’s day at their own quiet leisure before noticing envelopes bearing their names, telltale wax seals, and their precise locations where more typical addresses should be (i.e. “Emma Watson, Secondhand Bookshop”; “Matthew Lewis, Black Cab Stuck in Traffic”). As they open their invitations for the special, they play their parts with the same level of wide-eyed wonder that they brought to the first “Harry Potter” two decades ago.

Depending on whether you’re a longtime fan or a skeptical observer of the franchise, this wholehearted embrace of bottled magic may either thrill or bore you right off the bat. For anyone whose loyalties lie somewhere in between (as is the case for this viewer, a onetime “Harry Potter” obsessive whose devotion faded into adulthood), “Return to Hogwarts” manages to unearth enough genuine sincerity, and occasionally even jolts of pathos, to keep from becoming quite as sticky sweet as one of Harry’s treacle tarts.

The special traces the franchise’s journey through all eight movies, making callbacks to particularly beloved movie moments and staging recreations of elaborate sets for the actors to marvel at together as they reminisce. Watson sits down with co-stars Rupert Grint (Ron Weasley) and Daniel Radcliffe (Harry Potter himself) for familial talks about their inextricably linked childhood and stardom. Anyone who even vaguely followed the makings of these movies won’t be too surprised by many of the “revelations” that come to light, such as the well-trod facts that “Prisoner of Azkaban” director Alfonso Cuarón made the main trio write essays about their characters, or that Watson and Grint felt so much like siblings by the films’ end that their eventual kiss scene was, in Watson’s words, “the most horrifying thing either of us had to go through.” Even all this time later, they still know how to play their greatest hits.

With “Return to Hogwarts,” HBO Max has officially cornered the streaming market on nostalgic reunions of beloved franchises past. First it revisited “The Fresh Prince of Bel-Air” for a surprisingly frank conversation about the show’s legacy and controversies; then, the service made good on its launch promise to bring the entire cast of “Friends” together as a way of reminding everyone that “Friends” was gone from Netflix for good. Now that HBO Max has better established itself, it’s easy to take the cynical (and inevitably correct) view that this extravagant look back at one of the most successful franchises in entertainment history is mostly just WarnerMedia flexing its content library muscle for all to see. As was inevitable, “Return to Hogwarts” is a meticulous calculation as much as it is an emotional celebration. That it ends up charming as much as it does is a testament to its actors, game and charismatic as ever, once again finding a way to engross their audience in more intangible ways than merely following the script.

Besides the core trio, supporting players like Lewis, Tom Felton (Draco Malfoy), Alfred Enoch (Dean Thomas), Evanna Lynch (Luna Lovegood), and Bonnie Wright (Ginny Weasley) chat about their parts in the overwhelming “Harry Potter” whole. Interviews with those who played the adults in the room underline just how many members of the distinguished class of British actors found a place in this wizarding world — and, in melancholy moments that go unspoken until later on, those indelible presences like Richard Harris (Albus Dumbledore), Helen McCrory (Narcissa Malfoy), and Alan Rickman (Severus Snape) who have since died. I lost count of how many times someone insists the “Harry Potter” sets were “like a family,” and might’ve cocked a more skeptical eyebrow if the deep understanding between Radcliffe, Watson, and Grint in particular wasn’t so palpable.

For all those involved, one of the more intriguing aspects of the special is who isn’t included — namely, “Harry Potter” author J.K. Rowling. While there are some fond mentions of “Jo,” Rowling otherwise only appears onscreen in sparing excerpts from a 2019 interview. After spending the last couple years spreading baseless theories about trans people to the point that Radcliffe, Watson, and other cast members released statements affirming their support for the trans community, Rowling is quietly sidelined in this retrospective about how her work became an onscreen juggernaut. Her absence could’ve been enormously awkward, so “Return to Hogwarts” is smart to emphasize how these adaptations came together and impacted the world as blockbuster movies more than how Rowling’s vision inspired it.

For the most part, then, “Return to Hogwarts” focuses more on the physical filming process and emotional aftermath for its stars. There are further insights on the casting process, the brilliance of Stuart Craig’s production design and, inadvertently through behind-the-scenes footage of each film, a handy visual representation of how the early movies’ pride in practical effects morphed into reliance on green-screen CGI prove compelling on a pure level of “how did this even all happen?” level. When Radcliffe reunites with Chris Columbus, who cast and directed him as a child in the first two films 20 years ago, there’s something undeniably poignant about seeing him now able to interact with Columbus as a true peer. The same holds true for getting to see Radcliffe reunite with the singular Helena Bonham Carter who, much to Radcliffe’s utter delight, rips through the special like a mischievous tornado encouraging everyone else to follow her lead. (If this reunion were to have a host, she’d be the only choice worth making.)

So, yes, “Return to Hogwarts” is mostly aiming to envelop fans in a warm and fuzzy blanket of nostalgia. Nonetheless, there are a few revealing moments in which people admit just how overwhelming the entire phenomenon truly was: Grint choking up as he realizes how much his costars meant to him, Radcliffe explaining how growing up on this set became so “foundational” to who they became after leaving it, Watson remembering how “lonely” she felt after the weight of fame descended on her after “Order of the Phoenix” (as Felton, her childhood crush, emphasizes with protective fondness how “gracefully” she dealt with it). As calculated as this reunion is, these are surprisingly vulnerable admissions from people who have otherwise fiercely guarded their privacy for decades. Such bittersweet scenes, tucked in between the wistful smiles and footage of set hijinks, make for the special’s most compelling material by a mile. They can never “return to Hogwarts” as it once was, but they can still see and protect each other in a way that not one of the millions of people watching, despite this reunion’s best efforts to explain, will ever truly understand.

Harry Potter 20th Anniversary: Return to Hogwarts” premieres Jan. 1 on HBO Max.