Yeah, that's Attack on Titan. It's a really popular anime that is soon reaching its end.

If you don't wish to see it yourself, see this spoiler :) (click on this line)

The series takes place in a walled city known as Paradis Island, where the remnants of humanity reside. These massive walls were constructed to protect the people from the Titans who lurk outside. Inside the walls, life seems peaceful at first, but it's not long before chaos ensues.

A group of people from another country (that is more technologically advanced) come over to wreak havoc because they are afraid of their powers. This leads to a pretty intense plot that goes back and forth between various view points. You learn why the people from this Paradis Island have been oppressed, but also why this new country fears them. It ends up being, in my opinion, a well crafted story where you can never be sure who's an asshole and who's right. I do recommend you give it a try!

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Now, if you'd want to catch them by surprise, you could ask things like:

• "Do you think 3D Manoeuvre Gear would realistically work?" - could make a fun discussion topic that would be related to recent advancements in jetpacks, haha (if you and your son/daughter are interested in such topics)
• "Would you really leave the walls if you were thrown in that show?" - what the protagonists do is quite absurd (going out to fight these giants). See what he thinks about it :D
• A simple "Why do you like this show so much?" also works. Your son/daughter can then explain to you exactly what part of the show is their favourite, and perhaps that can lead to more things you can ask.

You can also attempt to catch up with them on the anime, and watch the finale together! It's supposed to be released in the next few months if I am not mistaken. That would also be a nice experience :)

Hope that helps! Don't hesitate to ask more questions if anything is unclear.