When Matt Smith bowed out of Doctor Who as the Eleventh Doctor in 2013, Peter Capaldi burst onto the scene as the Twelfth Doctor. Capaldi had made two previous appearances on Doctor Who before his official debut as the Doctor. He first appeared as the Roman Caecilius in 2008's "The Fires of Pompeii" and then in a pre-regeneration cameo as his Doctor in the 50th anniversary special, "The Day of the Doctor." When Capaldi made his proper debut, he introduced a very different take on the Doctor from his three predecessors, reintroducing elements of the First Doctor's grumpier and more detached persona.

Over the course of his adventures, Capaldi's Twelfth Doctor traveled with Jenna Coleman's Clara Oswald, Pearl Mackie's Bill Potts and Matt Lucas' Nardole. He briefly reunited with his time-traveling wife, Alex Kingston's River Song, and had a run-in with his very first incarnation, with David Bradley taking over the role from William Hartnell. As well as fending off classic foes, such as the Daleks, the Cybermen, the Zygons and the Master, the Twelfth Doctor also faced a range of new enemies, including the Boneless, a horde of killer spacesuits and a creature from his own nightmares, the Veil.

10 'Deep Breath' Introduces Twelve as a Ruthless New Doctor

The Twelfth Doctor's first full Doctor Who episode, "Deep Breath" saw the Doctor and Clara crash-landing in Victorian London, accidentally bringing a T-Rex along with them. Here, they reunited with the Paternoster Gang -- the Silurian detective Madame Vastra, her wife Jenny and the reconditioned Sontaran, Strax -- who helped the Doctor through the difficulties of his recent regeneration.

"Deep Breath" pitted the Doctor and Clara against the Half-Face Man, a part-man part-machine body horror creation, eventually revealed to descend from the same line of clockwork droids seen in 2006's "The Girl in the Fireplace." The episode saw Clara struggling to adapt to the Doctor's new appearance, while the Doctor had to rediscover himself. A mysterious conclusion left viewers guessing as to whether the Half-Face Man threw himself to his death or was pushed by the Doctor, leaving a question mark over just how threatening the Doctor had become.

9 'The Pilot' Sees Twelve Become a University Professor & Gain a New Companion

Doctor Who Season 10 gave the series something of a fresh start, following the departure of Jenna Coleman's Clara Oswald. The first episode, "The Pilot," revealed the Doctor was now teaching at St Luke's University, a job he had supposedly been doing for the last 50 years. The Doctor was now accompanied by Nardole, previously seen in Christmas specials "The Husbands of River Song" and "The Return of Doctor Mysterio." Crucially, however, "The Pilot" introduced new companion, Bill Potts.

Played by Pearl Mackie, Bill Potts worked in the university's canteen, but had been sneaking into the Doctor's lectures. The Time Lord recognized her potential and took her on as his student and, later, as his new companion. The episode saw Bill fall in love with fellow student Heather, who was then possessed by a sentient oil-like creature that needed a "pilot." Heather was bound by a promise not to leave without Bill -- a promise from which only Bill could free her in an emotional farewell.

8 'Death in Heaven' Reunites Twelve With the Master and the Brigadier

The Doctor Who Season 8 finale, "Death in Heaven," revealed the identity of the mysterious woman who had been lurking in the background throughout this season. Giving her name as Missy -- short for "Mistress" -- the main villain revealed herself to be the first female incarnation of the Master. "Death in Heaven" quickly demonstrated how ruthless a threat Missy was. She killed UNIT scientist Osgood and enacted one of the most unsettling evil schemes ever seen on Doctor Who: resurrecting the dead as Cybermen, after trapping and altering their minds in an artificial afterlife.

"Death in Heaven" saw the Twelfth Doctor contending with two old enemies at once, ultimately being forced to answer the question that had been haunting him throughout Season 8 -- "Am I a good man?" The Doctor, refusing Missy's offer of a Cyber army for him to command, simply declared himself "an idiot, with a box and a screwdriver, passing through, helping out." The episode also featured a touching final appearance from Brigadier Alistair Gordon Lethbridge-Stewart, resurrected as a Cyberman, as he came to the Doctor's aid one final time.

7 'The Zygon Invasion' & 'The Zygon Inversion' Reveal Twelve's Position on War

The two-part thriller, "The Zygon Invasion" and "The Zygon Inversion," resolved a dangling plot thread from the Doctor Who 50th anniversary special, "The Day of the Doctor." That episode had seen UNIT and the Zygons work together to reach a deal that protected both humans and the Zygons hiding among them on Earth. "The Zygon Invasion" revealed that this deal involved Zygons continuing to live, undetected, among the human population. However, not every Zygon was happy with this arrangement.

The story put the Doctor up against a Zygon revolution, with a Zygon splinter group seeking to wage war on humanity and those Zygons allied with them, in order to take Earth for themselves. The story also revealed that the Osgood killed by Missy was one of two -- the original Osgood and her Zygon double, though the survivor refused to reveal whether she was human or Zygon. The story culminated in one of the Twelfth Doctor's greatest ever speeches, as he called out the horrors, the pain and the utterly unnecessary nature of war.

6 'The Magician's Apprentice' & 'The Witch's Familiar' Reunite Twelve With Davros

The two-part Doctor Who Season 9 opener, "The Magician's Apprentice" and "The Witch's Familiar," not only brought back both Missy and the Daleks, but featured the surprise return of the Daleks' creator, Davros. The story saw the Doctor, Missy and Clara arriving on Skaro, in a gloriously retro Dalek city, modeled after the one seen in 1963's "The Daleks." After an encounter with Davros as a child, the Doctor is faced with a moral quandry he first proposed in 1975's "Genesis of the Daleks" -- whether he could kill a child if he knew that child would one day grow into a ruthless dictator.

The story features one of the greatest philosophical and personal stand-offs between the Doctor and Davros in Doctor Who history, with the two considering their similarities and the decisions they have made in their long lives. The episode also explored an unlikely alliance between Clara and Missy and featured just about every kind of Dalek to have ever appeared on Doctor Who, marking one of the series' all-time greatest Dalek stories.

5 'Oxygen' Sees Twelve Harshly Criticize Capitalism

The Doctor, Nardole and Bill found themselves on a space station where oxygen is sold for profit to the crew, via their spacesuits. Unfortunately, those suits are starting to malfunction, killing their wearers. Cut off from the safety of the TARDIS after the station detected the ship's unauthorized oxygen, the Doctor and his companions found themselves trapped in a nail-biting situation, relying on the dwindling oxygen supplies of suits that could turn on them at any moment.

"Oxygen" saw the Doctor being blinded, after giving Bill his spacesuit's helmet when hers was lost while navigating the station's exterior. The Doctor's blindness not only upped the ante of the episode, but persisted into the season's following two episodes. In its final moments, "Oxygen" delivered Doctor Who's biggest criticism of capitalism. It was revealed that the suits, which were capable of functioning autonomously, had been ordered to kill their wearers to save money. According to the Doctor, "The endpoint of capitalism. The bottom line, where human life has no value at all."

4 'Twice Upon a Time' Motivates Twelve to Regenerate into Thirteen

The Twelfth Doctor's final adventure saw him teaming up with the First Doctor. David Bradley, who had played William Hartnell in the Doctor Who biopic An Adventure in Space and Time, appeared in this episode as Hartnell's Doctor. Regular Doctor Who writer and actor, Mark Gatiss, joined the Doctors as a British Army Captain from the First World War. As all three men faced death, they were forced to consider their willingness to die and, ultimately, their reasons to live.

The episode offered an exploration of the impact the Doctor's adventures had had on time and space, examining how he became the hero fans know and love today. An emotional final scene saw Gatiss' Captain, revealed to be an ancestor of the Brigadier, saved from death by the Christmas Armistice of 1914. The episode concluded with Capaldi's Doctor summing up everything it meant to be the Doctor as he passed the mantle on to his thirteenth incarnation.

3 'Listen' Explores Twelve's Deepest Seated Fears

"Listen" dives into the Doctor's innermost thoughts in a way that Doctor Who has never quite replicated. The episode centers on the Doctor's growing obsession with the notion that if evolution can produce perfect examples of every survival skill, there must exist a species so perfect at hiding that nobody has ever encountered them. While the Doctor pursues this obsession, his efforts to bring Clara along with him clash with her first date with Danny Pink.

"Listen" is unique in that it is never revealed whether the Doctor's imagined creature exists. Every time he seemingly comes close to encountering it, other explanations can be offered up for the creature's apparent presence. The source of the Doctor's obsession is revealed to be a nightmare everyone has had -- a hand grabbing their foot as they get out of bed. The episode's end sees Clara hidden under the Doctor's childhood bed on Gallifrey, grabbing his ankle as he awakes, becoming the original source of this obsession. In a touching speech, she explains to the Doctor the importance of fear.

2 'World Enough and Time' & 'The Doctor Falls' Depict Twelve at His Most Vulnerable

"World Enough and Time" and "The Doctor Falls" marked the end of Doctor Who Season 10 and saw the Doctor bear witness to the genesis of the Cybermen aboard a Mondasian colony ship. While the first part delved into the body horror elements of the classic Cybermen, showing humans being gradually converted, the second part placed the Doctor in a grim battle that tested his resolve in impossible odds.

This story also saw the return of John Simm as the Master, a role he had last played in 2010's "The End of Time: Part Two," alongside Michelle Gomez's Missy. This was the first time in Doctor Who history that two incarnations of the Master ever appeared side-by-side. Both incarnations stayed true to form by killing one another when Missy betrayed her past self, resolving to join the Doctor.

1 'Heaven Sent' Succinctly Explored Twelve's Resilience

A masterpiece in spine-chilling horror, intriguing mystery and emotive character study, "Heaven Sent" is not only Peter Capaldi's greatest Doctor Who episode, but one of the series' greatest ever episodes. "Heaven Sent" saw the Doctor trapped in his Gallifreyan Confession Dial, a confessional device that had been turned into a personalized torture chamber by the Time Lords, after witnessing the death of Clara. Inside the Confession Dial, the Doctor was pursued by the Veil -- a creature taken from one of his most terrifying childhood memories.

As the Doctor struggled with the pain of losing Clara and tried to unravel the many mysteries of his prison, he gradually realized he had been there far longer than he thought. Over thousands of years, he had been producing new copies of himself from the teleporter that brought him there, burning up the previous iteration of himself for energy. At the very edge of the Confession Dial was an outer wall twenty feet thick and four hundred times stronger than diamond. Over billions of years, iteration after iteration of the Doctor punched through the wall, in the ultimate display of dedication and refusal to surrender.

Doctor Who returns to BBC iPlayer and Disney+ on May 10.