Mark Grayson goes to Hell in episode four of season four, and honestly? Good. He needed it.
After three episodes of watching Mark give himself an ulcer about whether killing is ever the right thing — and one trip to actual Hell where Satan himself more or less confirmed that yes, sometimes it is — he surfaces wearing his old suit and smiling for the first time in what feels like most of this show's run. "Hurm" is about as close to a fun filler episode as Invincible gets, which means Damien Darkblood is back, Bruce Campbell is playing Satan, and Mark gets to spend forty minutes punching a lava monster while the animation team goes feral with the color palette. There are worse ways to take a breath.
The breath matters because the show earns it. Mark's been carrying the weight of a morality problem with no clean answer through some genuinely grueling television, and the trip underground gives him a contained version of the question — here's a monster, here's the stakes, here's why stopping her matters — where the calculus is just clear enough that he can bank some resolution. He comes back up a little lighter. Ready to deal with the pregnancy conversation he and Eve keep almost having.
And then Nolan arrives.
So: yes, it's a detour. It's also exactly the shape the season needed at exactly this moment. The Viltrumite War is coming, Growth Era Dad is back and apparently on the right side of it, and somewhere down the line Oliver's going to need a partner whose powerset is whatever happens when Atom Eve's kid inherits the family business. There's a lot of good television still ahead. "Hurm" was a nice place to stop and remember that Mark can still punch something and smile about it.