Episode 18 of The Witness picks up in the marsh, at the block-shape panel that switched on at the end of the last video. Basket finishes the row, meets a new symbol in a dark room, turns a bridge, and ends underwater with two panels of the new kind solved.
The last marsh panels
The third orange panel holds several small block shapes, and one of them is fixed: "So this is one we can't rotate because it's not tilted." Line after line goes in until a white line runs through the grid and the next panel switches on.
The fourth takes even longer. "These things together actually are just a 2x3," the commentary works out, and later counts the pieces: "I have the L, I have the 2x3, and I have the 1." The line goes in, the panel lights, and a yellow cable glows off toward the water. "Yes! Okay, that's what I was thinking, is that it might actually drain the water out of these little paths."
New rules
Down a walkway and into a dark building, a narrow gray panel shows a small blue symbol beside yellow blocks. "New rules. So. What? Is this subtraction?" The commentary settles on "this shape is actually a negation shape, I think." The panels grow from thin strips to 4 by 4 grids, and each takes a blue line: "the shape is a one by four, but then I subtracted two from it." Five panels later: "Okay, that wasn't so bad. That's only phase one of this."
A bridge panel
Back outside, past purple water, a square white panel stands among the walkways with pink, blue, yellow and red marks on it. "This is another puzzle, but it actually changes the bridge," the commentary says, and the bridges are "color coded." The line drawn connects "red to the gray," which the commentary reads as rotating the piece of walkway underfoot: "eventually it will let us cross here."
Stairs lead down to a teal room with a window under the water, where a small red panel takes a line. "Oh, we can shortcut again." The commentary guesses the earlier panel "drained the water from here. So now here I'm guessing more negation."
Negation underwater
It is. The first gray panel in the underwater room goes blue after about a minute. The second does not. One attempt fails because "we negated too much." "This is so bizarre. This is really weird." For over three minutes the commentary tries to fit two shapes of four together and take one square away.
Where it ends
"So we have straight across, and then they cross, they overlap in the middle there, but we have negation so we can actually subtract one of those." The line turns blue and the panel is solved. A third panel with its own shapes waits beside it, untouched, and the video ends there.
