Though Doctor Who is a science fiction series that regularly reinvents itself, certain things are constant. The Doctor is the Doctor, no matter who is playing the character. She travels through time and space with a variety of human companions. She battles Daleks and Cybermen, and is fascinated by the innate wonder of the universe. And the Master will always be her greatest enemy. Or best friend. Or both, depending on who you ask.

A fellow renegade Time Lord, the Master is one of the few consistent elements of the Doctor’s long life, no matter her regeneration. Her first friend, her school chum, her "man crush," the Master is part adversary, part dark reflection, and part hopeless quest. Their relationship is basically the definition of messy, but their connection is undeniable. "A friendship older than your civilization. And infinitely more complex," is how Missy — a female incarnation of the Master — explains their relationship to Twelfth Doctor companion, Clara. And she’s not wrong.

The two Time Lords have fought one another, saved one another, and virtually everything in between over the course of Doctor Who's long history. It’s almost impossible to imagine one without the other, which is probably why their relationship remains so fascinating after all these years. And Season 12 is no different.

Though the return of the Master in Season 12 premiere "Spyfall" came as a shock to many fans, the relationship between Thirteen and her suddenly regenerated best frenemy still feels as organic and fundamental to the series as ever. The Doctor and the Master may have both changed faces since last they saw one another, but their longstanding connection continues to drive the series forward in the same way it always has.

Each Doctor/Master pairing has a slightly different tone and flavor to it, much as each incarnation of the Doctor is an evolution of the one that came before. Certain underlying truths remain throughout, of course — the Master is jealous and possessive; the Doctor thinks he might still be worth saving. The Master makes plans for world domination; the Doctor thwarts them. They argue about the future, and often relive the past. But the end of the day, one stark fact remains: Both, in their very different ways, care deeply about what happens to the other. ("If somebody kills you and it's not me, we'll both be disappointed," Missy declares at one point.) It’s just the packaging around the relationship that changes.

Earlier in the reboot era, Tenth Doctor David Tennant and his Master, John Simm, together embody the despair and desperation of two people who truly believe they are the last of their kind. Simm’s Master, driven mad by the sound of drums the Time Lords embedded in his head, is an intriguing combination of manic and cruel, a character who both wants to punish and reclaim the Doctor for his own. Their relationship feels like nothing so much as bitter exes who can’t quite let go of whatever’s between them, and it’s easier than ever to see how the Master is the man the Doctor could have become if only his life had gone differently.

The two are wildly dysfunctional as well as strangely appealing together, and when Simm’s Master allows himself to die simply to spite his former friend, well. It feels like that’s probably the only way the story between them could have ever ended.

Until... it isn't.

When the Master next returns — inexplicably, of course, but that’s how he rolls — he’s the Mistress, a frighteningly dark Mary Poppins-esque figure played by Michelle Gomez. The relationship between Missy and Peter Capaldi’s Twelfth Doctor initially mirrors many previous Doctor/Master pairings, where she does her best to tempt her old chum to the dark side. But when the threat of execution leads Missy to beg her former BFF for a chance at redemption, it’s an entirely new and unprecedented sort of story.

As Twelve helps Missy find something like grace over the course of Season 10, the two grow closer than ever — a shift in their relationship that changes them both in the process. So much so that Missy ultimately chooses the side of light when it counts, ultimately rejecting the evil entreaties of her former Simm incarnation and standing with the Doctor at last.

That she is killed for it (by that same earlier self!) and dies without Twelve ever knowing what she tried to do is painfully tragic, but also an ultimately perfect end for this Master, who finally became the person the Doctor always beloved she could be. (Even if we’re the only people who ever saw it.)

Most Whovians knew that this wasn’t the end of this long-running character, though many were undoubtedly surprised when the Master reappeared just two short seasons later, played by Sacha Dhawan. A much more manic and unstable version than his predecessor, this Master is all rage and sharp edges in a well-cut suit, exhibiting the sort of sustained, non-directional fury that burns down whatever comes near it. Yet, there’s a certain nihilistic, despondent quality to him as well, evidenced by his genuine apathy towards the prospect of his own death and the overly performative nature of his villainy, which plays to the back row at all times. (One gets the sense that the idea of "being" the Master is largely a mask that this incarnation wears, in a way that his previous selves have not.)

What happens next for the Doctor and the Master is a mystery, but one that's both exciting and familiar. When it comes to this duo, the more things change, the more things stay the same.