Doctor Who: Every Modern Companion's DEFINITIVE Episode

If you're trying to define Clara as a companion in a single story, the Series 8 finale is the one. This is the moment that her complex companion dynamic is pushed to its breaking point.

What makes Dark Water so compelling is that it doesn't soften Clara's flaws to make her sympathetic. Clara deceives and betrays the Doctor in the opening moments, and this isn't framed as a minor lapse in judgement or a misunderstanding. It's a calculated and selfish betrayal, and probably by far the worst that any modern companion has committed.

She lies to him, manipulates him, and ultimately attempts to threaten him to get her way, to the point where she's willing to doom both of them if he doesn't comply. Clara, in this story, is selfish, reckless and controlling (even if her motives are sympathetic), and Moffat doesn't sugarcoat it.

These glaring flaws in her character are what makes Clara so interesting, and while they might rub some fans the wrong way, they're a compelling break from the norm. In Capaldi's era, she is rarely his moral compass, and is far closer to mirroring the Doctor than any companion we've seen prior. In this two-parter she even goes so far as to convincingly impersonate him (and steal his name position in the credits) to really hammer the point home.

She's gone through her period of outrage at the way Twelve conducts himself, and has come round, dangerously so, to his callous and cold way of thinking. In an odd twist, it's actually the Doctor who is forced to pick up the emotional slack in this one, forgiving Clara in spite of her betrayal.

At the same time, the story sees the climax of Clara's relationship with Danny Pink, which is handled here with an unusual finality for the show in the form of a "boring" car accident. Danny’s death isn't reversed (though there's a little subversion in the story with it looking like it will be undone). This is a huge moment in Clara's arc, and her grief directly drives her character for the remainder of her time on the show, with her recklessness ultimately being her undoing.

This is pivotal character development for Clara. Her complex relationship with the Doctor is what she's most remembered for, and you can't get a better case study for why that pairing is bad news than this story right here.