If there’s a saving grace to Doctor Who’s messy and bloated previous episode, it’s that it set up the last of the narrative dominoes for “Ascension of the Cybermen” to knock them all down in impeccable style. The first part of series 12’s two-part finale balances its characters, plot threads and villains near perfectly, raising tensions so high that the wait for its conclusion next Sunday will be nearly unbearable.

Following coordinates given by the historical poet Percy Bysshe Shelley (Lewis Rainer) in the previous episode, the Doctor (Jodie Whittaker) and her companions arrive on Earth in the far future where all but seven humans remain from conversion into the emotionless Cybermen (Nicholas Briggs). It’s a desperate state of affairs, and things only get more desperate as the story draws on.

Leader of the Cybermen is Ashad (Patrick O’Kane), who also made his debut last episode. Previously dubbed “the lone Cyberman,” the human-Cyberman hybrid is now anything but, with armies of them at his command. Even for the Doctor, the villain displays both a cruelty and cunningness rarely seen in Doctor Who, with O’Kane giving a chilling performance aided by his unsettling design.

Aided by an even split in the main cast early on, with Graham (Bradley Walsh) and Yaz (Mandip Gill) escaping with most of the remaining humans and the Doctor and Ryan (Tosin Cole) confronting Ashad, everyone gets a chance to shine. Graham, in particular, shows real development by confidently leading the group, while the Doctor delves deeper into her darkest self, becoming consumed with defeating Ashad.

The episode intermittently cuts to 20th century Ireland, following the life of the seemingly normal Brendon (Evan McCabe) from infancy to young adulthood. Things of course take a turn for the supernatural near the episode’s conclusion, but the mystery surrounding this new character only makes this season’s plot more intriguing.

Even the guest cast is excellent, with Earth’s remaining humans selling the apocalyptic setting with an intense desperation that eventually leads to hopelessness as things get even more dire. Doctor Who has dealt with “end of the world” threats countless times, but the survivors’ interaction with the main cast makes the conflict more personal.

“Spyfall” and “Praxeus” director Jamie Magnus Stone and lead writer Chris Chibnall keep up the tension so well throughout the episode that despite the standard 49-minute runtime, the time flies by. Despite some duds along the way, series 12 has been such a concoction of captivating villains and intriguing mysteries that it’s been impossible to look away. It’s now up to the finale to deliver on the lofty setup that’s been given.

The Doctor Who series 12 finale airs Sunday at 8 p.m. on BBC America.

@JosephStanichar

js080117@ohio.edu