In the Doctor Who of old, companions would come and go at the drop of a hat. Sometimes they weren't even afforded a proper exit and faced the indignity of being trundled away off-screen.
Since the show's big comeback in 2005 though, the personal and emotional lives of the Doctor's fellow TARDIS travellers have been a crucial part of the series, and as a result their departures have become more epic, more thoughtful, better structured.
'The Power of Three' is perhaps best enjoyed as a prelude to epic finale 'The Angels Take Manhattan'. This seventh series of Doctor Who has been anchored by what's clearly a carefully-considered story arc and it's fantastic that Steven Moffat and his team of writers - in this case, 'Dinosaurs on a Spaceship' scribe Chris Chibnall - have gone to such lengths. Barring any major slip-ups next week, the departure of Amy (Karen Gillan) and Rory (Arthur Darvill) could well be the best-executed companion exit this show's ever had.
The Ponds' penultimate adventure boasts an intriguing premise - we often hear about how companions are the audience surrogate and how the story of the Doctor is told through their eyes, but this is the first episode to really live up to that spiel.
As Amy's voiceover suggests, this is the one in which the Doctor becomes a part of his companions' lives, rather than the other way around, and the Time Lord finds domestic life - jobs, a house and other human concerns - confusing and frustrating.
Steven Moffat may have claimed in the past that the Doctor and Sherlock Holmes actually have remarkably different personalities, but in many ways 'The Power of Three' shows our hero at his most Sherlockian - crushingly bored when not unravelling a mystery.
It's clear that his constant desire to escape from 'mundane' everyday life is beginning to grate on the Ponds and, more than that, their time-and-space escapades are actually negatively impacting their 'real' lives. No companion has ever really attempted the half-in/half-out lifestyle before and there's a reason for that - it simply doesn't work.
It's interesting that Brian Williams (the returning Mark Williams, still brilliant) seems to be the only one who's still touched by the Doctor's magic. Ten years after 'The Eleventh Hour', Amy and Rory are seasoned travellers and at times 'The Power of Three' seems to suggest that companions will inevitably grow up and move on from the Doctor - he's essentially Peter Pan.
Of course, the Doctor's a canny sort and fully aware of Amy and Rory's conflict of interest, leading to a touching heart-to-heart between the Raggedy Man and Amelia Pond. There's the weight of Matt Smith and Karen Gillan's own goodbyes behind the words of the character they're playing here.
We've yet to mention what is technically the episode's A plot - the "slow invasion" or the "invasion of the very small cubes", which leads to panicked media news reports and a string of random celebrity cameos in scenes heavily reminiscent of the RTD era.
Even old favourites UNIT get involved, and guest star Jemma Redgrave immediately endears herself as Kate Stewart, who's taken on the Doctor's old role as the military organisation's scientific advisor. And following on from the touching tribute to Nick Courtney in last year's 'The Wedding of River Song', we get another lovely moment here when Kate's revealed as the Brigadier's daughter and makes clear the true impact the Doctor had on her father's life. To paraphrase a line of the Doctor's from last week, she can stay.
But as months pass with no activity or apparent threat, the cubes are soon forgotten and integrated into society. Once they finally turn deadly, they're are an intriguing enough threat, and the revelation that the cubes are intergalactic 'slug pellets' is certainly clever, but in all honesty neither they nor the force behind them are really important. At its heart (or should that be hearts?) 'The Power of Three' is a character piece about friendships and feelings.
So it almost feels like a shame when the cube plot thread finally kicks into gear at the episode's mid-point. It's all reasonably diverting but oddly nowhere near as engaging as the episode's more 'ordinary' first half. And top actor Steven Berkoff is wasted in the all-too-brief role of chief villain Shakri.
Still, 'The Power of Three' remains an emotional, fun and involving Doctor Who episode, one in our opinion that's miles ahead of Chibnall's recent 'Dinosaurs'.
As the story draws to a close, the Ponds put aside growing up and ordinary lives once more to travel with the Doctor, but it's hard to feel merry when we know this latest jaunt will be short-lived. Will Amy and Rory leave the Doctor or be torn from him? We've only a week to wait before we know for sure...
