You can watch this episode right here.
We learn that Whitestone is an actual type of stone. When it comes in contact with residuum, both shatter or explode.
The adventurers are concerned that Percy is a bit unhinged. To put them at ease, he tells them a bit about his life after the Briarwoods killed his parents. He climbed onto a sailing ship and ended up homeless in some other city.
Keyleth, a druid, senses that the roots of the sun tree seems to be connected to some kind of necrotic energy.
Archie thinks that Percy being back could inspire the people who remain in Whitestone. He explains that Cassandra is the prized hostage of the Briarwoods.
Scanlan convinces the group to let him make a big distraction while the group tries to rescue Cassandra.
Going off alone, he tries to start a fire, but it begins raining. Creeping into a house, he ends up in a room where the mid boss with the golden arms is going to the bathroom.
Guards rush him, but he drinks a potion that allows him to teleport about and avoid their attacks. When that wears off, he drinks a potion of diminution, and after that, a potion that gives him indigestion. He is captured briefly, but it turns out to be a potion of fire-breathing. The entire place catches fire. That was so good.
He flees into another room and uses a scroll to polymorph himself into a triceratops.
The mid boss catches up with him on the roof of the burning building. Scanlan actually fights using a Bigby's hand, punching the dude repeatedly. He ends up firing a bolt of energy from his groin and floats away on Bigby's hand (which I think he is referring to as "Scanlan's Hand").
Seeing the guards of the city running to the fire, the rest of the group makes their way to the manor of the Briarwoods. Guards are ready for them.
Percy makes his way into a room where he finds his old tutor, Anders, fighting with Cassandra. Anders slits her throat.
Thoughts
This was a Scanlan-focused episode, which is fine. It's a lot of fun to see D&D stuff on the screen - spells, potions, everything.
Also... residuum! Residuum is some classic 4e stuff, did not expect to see it here on this show.