Since I'm late as always I decided not to quote anything and keep my response short... as short as possible.

Overall, the episode had potential but... then really went downwards.
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I won't agree with it needing to be a two parter. I claim they should have removed the threat part entirely and just made it a happy go lucky episode with the cubes being... a prank or something.

I loved the character interactions (even though... I REALLY can't believe Bryan kept surveying the cube for one year. That's way too long) and what it did about their lives and feelings and... stuff. Also, I liked quite a few of the hijinks and jokes in it (like "You hover. I had a metal dog that could do that.")

The cubes were interesting but... everything in the end and about the revelation was bad.
Okay, admitting maybe they abducted people to study them, stopping their hearts is so overly complicated... and fixing the hearts after five or more minutes, probably much more is not a proper way to save a life... urgh. The alien race... came out of nowhere to destroy humanity and then disappears again? Well, maybe we will see them again. The people getting left behind... I'll accept the Doctor not expecting the ship blowing up (maybe) but it still leaves a poor taste in my mouth.

No, really, I'd much rather have them use a different ending to the story... but it was still fun and that only spoiled like the last ten minutes... It was a fine episode, just not great.