The title card reads "S1E23," with an on-screen credit "with Precisi." Basket had been "doing some AFK collection of some gunpowder," and on the way back to the main base, "I noticed a wither health bar show up." Basket reports "a big hole right there" in the side of the building and isn't dressed for a boss fight: dyed leather armor and not much else.
A suspect
Basket already has a name in mind. Sir_Cadbury had killed Basket inside the base before, and had talked about withering it. The chat later backs that up, in Sir_Cadbury's own words: "i sad i'd wither you yesterday" and "and then this happend." There had also been missed calls. The chat shows "Incoming call from Sir_Cadbury, pickup a smartphone to answer."
Basket had "backed up a little bit so that it doesn't follow me," takes a look at the building from outside ("Looks like there might be another hole there too") and then sneaks over to see what's left.
Sir_Cadbury kills it
By the time Basket gets inside, the job is done. Sir_Cadbury types "BASKEEET," then "i killed it," then "i swear it wasnt me :0." Basket types "thanks for killing it! :D," and Sir_Cadbury hands over the drop. The tooltip reads "Nether Star." Basket's reply: "nice! I can make a beacon :D"
Sir_Cadbury's version of events, in chat: "i logged off on the roof" and "and i logged back on an hour later and it was there." On the call, the beacon idea gets a second look. It "makes a big pillar of light," and "people would find our base pretty easy." The answer: "Well, we could use it for our next base."
The damage
"Oh my gosh, there's holes in our base." Then: "The staircase is gone." Precisi turns up in chat with "Cad!" and "Our base!" Basket types that "the lounge and the creeper water park are fine," and finds one thing worth keeping: "at least our sign is still here cads," the one marking "when you chopped off my heada <3."
It isn't only the wither. "Oh, somebody let all the villagers out." "Oh, the cows are all dead." "All of our chests are empty up here." But "they didn't find any of the hidden chests though," so the plan is to leave those alone and let people find them someday. Sir_Cadbury asks "do you have any enimies?" and Precisi answers "Not specifically."
A hill full of signs
A voice on the call spots "a bunch of signs" on a plateau, "the one where you were using to get the zombie villagers." Basket climbs up and finds a crowd of them: "Who puts all this signs up?", "well, this escalated quickly", "STOP PUTTING SIGNS UP!!!" and "finally! I made it! The top of the mountain!" Basket hunts for one claiming the wither, "but I guess not."
Presents and the end of a building
The guest house got the opposite treatment. Someone put up a painting ("I like the, uh, redstone carpet in here" too), and the guest book has new entries. Then Sir_Cadbury types "psst" and "a present for your loss," and three diamond blocks land in Basket's inventory, along with "tell me if you find out who it was" and "AND THEY SHALL DIEEE." On the call: "I don't think he did it."
Basket's guess is that "they kill all the chickens and the cows and the sheep," but "they didn't clear all the eggs," so Basket grabs some. And "we still have the two ender chests that are hidden." Nobody wants to rebuild. "I think we should call this the end of first base. Or at least the end of the first building." The guest house, the lounge and the Creeper Water Park are still standing. The episode ends on the end card, with no gameplay after it.