The British sci-fi show is celebrating its 60th anniversary with three specials featuring some familiar faces.It’s rare for a television show to celebrate its 60th anniversary. It’s even rarer for a show to be entering a new era on its 60th anniversary.But “Doctor Who,” the British sci-fi show that began airing on the BBC in 1963, is in a period of expansion. Three upcoming specials, celebrating the show’s latest milestone, will arrive weekly on Disney+ in the United States from Saturday, as part of a deal between the streamer and the BBC.And then a new season, starring Ncuti Gatwa (“Sex Education”) in the title role, will arrive next year on Disney+ (and the BBC in Britain) following an extra Christmas Day episode. Russell T Davies, who relaunched the show in 2005, is the showrunner for them all.“Doctor Who” has decades of adventures, villains and complex story lines for dedicated fans to immerse themselves in. But if you’re new to the show, here’s what you need to know before tuning into the upcoming specials.A Quick RecapDavid Tennant, right, as the Doctor in Season 4 of “Doctor Who.” Tennant will rejoin the show for the 60th anniversary specials. Adrian Rogers/BBCThe Doctor is a Time Lord from a planet called Gallifrey, who travels across time and space in a Tardis, an unassuming spacecraft that looks like an old British police box, which members of the public used to call the authorities. His mission is to protect Earth, and the humans who live there, from a variety of threats.“The Doctor is the nerd, the well-read misfit, who isn’t particularly physical, who still wins the day,” said Toby Hadoke, an actor who hosts a podcast dedicated to the show. “The Doctor always offers hope for the person who feels slightly left out.”David Tennant, who played the Doctor between 2005 and 2010, and will be back as the star of the 60th anniversary specials, said that he thought the show’s appeal was “the way the domestic and the simplistic and everyday meets the fantastical and the absurd.” In the show’s world, “the most extraordinary things become very relatable,” he said.The show’s longevity is partly thanks to the fact the Doctor can “regenerate,” meaning a new actor can step into the role, but the show also experiments with genre, and the same season can include a historical drama one episode and a modern political satire the next.“Every time the Tardis door opens and the team steps out to a new planet, or a new time, or a new story, then it begins again,” Davies, the showrunner, said.The Doctor usually travels with a regular human companion, who in the 60th anniversary specials is played by the comedian Catherine Tate.Where Are We With the Plot?Jodie Whittaker became the first woman to play the Doctor in Season 13.BBCAt the end of the last season, Jodie Whittaker, the 13th incarnation of the Doctor, regenerated.Traditionally, a new actor plays each incarnation, and Gatwa is confirmed to be the 15th Doctor. But for the upcoming 60th anniversary episodes, Whittaker has turned back into Tennant, who was the 10th Doctor from 2005 to 2010, and then again for a 50th anniversary special in 2013.Rather than reprising the 10th Doctor, in the upcoming specials, Tennant will portray a 14th Doctor, the first time an actor has played two distinct Doctors. (Keeping up?)“Who is to say you can’t do this?” Davies said. “There’s absolutely no doubt that it can happen.”Tate will also reprise her role as Donna Noble, the Doctor’s companion. But in their last adventure together, which aired in 2008, the Doctor wiped Donna’s memory, and with it all recollection of their time together. If Donna remembers him, she will die. And yet they will reunite in the upcoming specials.“I had left our heroes in a tragic situation separated forever, unable to ever be happy again,” Davies said. “That’s begging for a final act, isn’t it?”How to Watch in the U.S.Ncuti Gatwa will star as the Doctor in the show’s upcoming season.Tolga Akmen/EPA, via ShutterstockWhile “Doctor Who” has aired in the United States for a number of years, including on PBS, the Sci Fi Channel and BBC America, the new international distribution deal with Disney+ could make the show more accessible to a casual audience. For new viewers, the 60th anniversary specials will begin with a prologue recapping the Doctor and Donna’s story.If you would like to dive deeper into the back catalog, older “Doctor Who” episodes are available to stream in the United States on Max or BritBox.An Inclusive Sci-Fi ShowYasmin Finney will join the cast of “Doctor Who” in the new season.Gareth Cattermole/Getty Images“Doctor Who” has long been notable among sci-fi franchises for its onscreen diversity. Whittaker became the show’s first female Doctor in 2017, and in 2020, Jo Martin played an incarnation of the Doctor known as the Fugitive Doctor, the show’s first Black doctor. And Yasmin Finney, a trans actor who played Elle in the Netflix show “Heartstopper,” is also joining the cast.“The show has always been good at appreciating inclusivity, and cherishing the different,” said Tennant, who added that he grew up as a “skinny bloke with specs in Scotland, who didn’t feel like the coolest person in the room.”But “the Doctor celebrates uncoolness,” he added. “And that was something I appreciated.” More