The episode also saw the return of UNIT and introduced their new scientific advisor, Kate Stewart (Jemma Redgrave), daughter of Brigadier Lethbridge-Stewart.

When they reach zero, the cubes open, but appear empty; however, massive numbers of cardiac arrest cases are reported, killing about a third of humanity.

As Earth recovers, the Doctor prepares to leave alone, but Brian insists he take Amy and Rory on a once-in-a-lifetime adventure, asking "just bring them back safe".

[10][11] Chibnall described "The Power of Three" as "a lovely big Earth invasion story" but different from the ones done before, as it focused on Amy and Rory's time with the Doctor and the impact of him on their lives.

[9] Matt Smith put disgust into the Doctor's remark concerning Twitter in the episode, reflecting his real-life decision to stay off the social network.

[12] At Chibnall's request, "The Power of Three" sees the return of UNIT, which first appeared in The Invasion (1968) and became a regular feature during the Third Doctor (Jon Pertwee) era.

[13] The episode reveals that Kate Stewart is now running UNIT; she is the daughter of Brigadier Lethbridge-Stewart and previously appeared in the Reeltime Pictures direct-to-video films Downtime and Dæmos Rising played by Beverley Cressman.

[16] Their last scene filmed together was getting into the TARDIS with the Doctor after saying farewell to Brian;[1] when the doors closed Gillan, Darvill, and Smith hugged and started crying.

[7][19] Producer Marcus Wilson stated that a "hundred" individual cube props were made, with "many more" added with computer-generated imagery (CGI).

Dan Martin of The Guardian stated he "bloody loved" the episode, calling it "a nostalgic run through all the best bits of the Russell T Davies era".

However, he noted that it "also had the weaknesses of some of [Davies'] adventures – the ending was so underdeveloped that even a magic button couldn't explain it – but 'The Power of Three' was, in every sense, completely gorgeous".

[4] Neela Debnath, writing for The Independent, praised the way the episode showed the companion's life outside the TARDIS and celebrated Amy and Rory, as well as the introduction of Kate Stewart and her connection with the Brigadier.

[27] Radio Times reviewer Patrick Mulkern described it as "beautifully made television" and welcomed Kate as a "wonderful addition".

Despite noting the plot was "fairly standard" and "easily foiled", he wrote that the concept of the cubes and "slow invasion" was "a cleverly executed bit of business".

[30] The Daily Telegraph reviewer Gavin Fuller gave it a score of two and a half out of five stars, feeling that it was "treading water" as a lead-in to the following finale.

However, he enjoyed Amy and the Doctor's conversation and the "countdown element and the mystery of the cubes", but found the explanation unoriginal and the conclusion too easy.