What the Shell

What the Shell is a simultaneous-turn strategy game: both players move at once, and neither sees the other's move before committing their own. The easiest way to learn is the interactive tutorial in the app — but players kept asking for the complete written rules in one place. Here they are: every rule of the game, nothing left out.

Object of the Game

Line up five of your turtles in a row — across, down, or diagonally — and stop your opponent from doing the same. A row of five ends the game. (If both players get there on the same turn, see Sudden Death.)

The Board & the Pieces

What the Shell is played on a 9×9 grid. One player commands the black turtles, the other the white.

Black turtle

White turtle

Egg

Eggs appear on the board when both players collide on the same space. An egg belongs to no one — until it hatches.

How a Turn Works

Who goes first? The player who most recently jumped into a pond and transformed into an egg goes first. So actually, nobody goes first — every turn is simultaneous.

Each turn, both players secretly choose any empty space, and both moves are revealed at the same time. Turtles never move once placed; they only leave the board by being captured. Each turn resolves in this order:

Placement. Both turtles land on the board.
Captures. Any pieces bracketed by the new arrivals are removed.
Hatching. Each capture hatches a waiting egg into the capturer's turtle.
Victory check. Rows of five are counted only now, after everything has settled (see Winning and Sudden Death).

Collisions & Eggs

If both players choose the same space, neither turtle lands there. The collision leaves an egg instead.

An egg is neutral: it belongs to neither player, and it blocks its space.
No captures happen on a collision turn — no turtle arrived, so nothing can bracket.
Eggs stay on the board until they hatch (see Hatching) or are captured.
The board remembers the order eggs were laid: the oldest egg always hatches first.

Both players wanted E5. An egg lands there instead.

Captures

You capture by bracketing: when your newly placed turtle traps exactly two enemy pieces in a straight line against another turtle of yours, those two pieces are removed from the board.

The bracket can run in any of the 8 directions — across, down, or diagonally.
The two trapped pieces can be enemy turtles, eggs, or one of each. Yes, you can capture eggs.
Exactly two: a lone enemy piece, or a row of three, cannot be bracketed.
One end of the bracket must be the turtle you just placed (or just hatched); the other end can be any of your turtles already on the board.

Every capture you make hatches one egg into one of your turtles (see Hatching) — a bracket removes two enemy pieces but hatches only one egg. A single placement can even capture in several directions at once — each direction is its own capture, and hatches its own egg.

Black plays E5, trapping two white turtles against the black turtle on H5. Both are captured.

Walking into a trap is safe. A capture only triggers when the bracket closes from the outside. If you place your own turtle into the middle of an enemy bracket, nothing happens.

White plays E5, completing the pair between two black turtles — and is perfectly safe. A bracket only snaps shut when the turtle on the outside arrives.

Captures are simultaneous too. Both players' captures are worked out before anything is removed, so both can capture on the same turn — and a turtle that is being captured still completes its own capture. Mutual captures both happen.

Anticipatory captures. Placements happen before captures are checked, so a turtle your opponent places this turn can be trapped by your move on the very same turn — if you predict where they'll play, your trap snaps shut the moment their turtle lands.

Hatching

Capturing doesn't just remove enemy pieces — it hatches eggs. Immediately after captures, still in the same turn:

Each capture you made hatches the oldest egg on the board into a turtle of your color.
A freshly hatched turtle can capture, exactly like a placed one — and that capture hatches the next egg. Hatching can cascade.
If both players captured, the oldest egg is claimed by both at once and dissolves: it disappears, hatching for neither.
Hatching continues until the captures — or the eggs — run out.

Captures are never saved up for later: if no egg is waiting on the board when you capture, nothing hatches — not now, not next turn. Tip: Time your captures for when an egg is on the board.

Black captures two whites on the G-file. As a reward, the waiting egg at C3 hatches — into a black turtle.

A big turn: black's E5 captures four turtles in two directions, while white's B4 captures two. One capture from each side cancels on the oldest egg — egg 1 dissolves. Black's remaining capture hatches egg 2 into a black turtle, and egg 3 stays on the board.

Winning

After each turn has fully resolved, check for rows of five or more turtles of one color — across, down, or diagonally. If you have a row and your opponent doesn't, you win. (Both made a row on the same turn? See Sudden Death.)

Victory is checked only after captures and hatching settle — a row that forms and is broken within the same turn never counts (see Anticipatory Captures).

Five black turtles from D7 to H3. If white can't match it this turn, black wins.

Sudden Death

Both players complete a row of five or more on the very same turn? Because turns are simultaneous, it happens. When it does, count each player's winning turtles: every turtle that is part of a row of five or more. A row of six counts six; two crossing rows of five that share a turtle count nine (shared turtles count once). If one player has more winning turtles, they win immediately.

Five against five on the same turn — Sudden Death. Play continues until one row breaks.

If the counts are exactly equal, the game doesn't end — it sharpens. Play continues in Sudden Death: both rows stay on the board, winning turtles are recounted from scratch every turn, and the moment the counts differ, the higher count wins. Three ways to tip the balance:

Extend your row — a row of six beats a row of five.
Make a second row — two rows of winning turtles beat one.
Destroy their row — capture turtles out of it to shrink their count.

As long as the counts stay equal, Sudden Death continues. And if captures break both rows on the same turn, nobody wins: normal play resumes, and the game can even enter Sudden Death again later.

Counting winning turtles: white's two separate rows hold ten turtles, while black's crossing rows hold only nine — the shared turtle counts once. White wins, 10 to 9.

The longer row isn't the bigger count: white's unbroken row of eight holds eight turtles, but black's cross holds nine. Black wins, 9 to 8.

Draws

If the board fills completely and neither player has a row of five, the game is a draw. In practice, draws are extraordinarily rare.

Quick Reference

Both players place at once. Same space → an egg.
Bracket exactly two enemy pieces (turtles or eggs) between your turtles → captured.
Each capture hatches the oldest egg into one of your turtles — immediately.
If both players capture on the same turn, the oldest egg disappears instead of hatching.
Captures never carry over: no egg waiting, no hatch.
A row of five or more turtles wins.
Both players complete rows on the same turn → Sudden Death rules decide. Full board with no winner → draw.

I hope you enjoy playing What the Shell as much as I've enjoyed making it, going all the way back to its origins in a little game called Five or Ten for QBasic in 1996 (with only 3 rule changes since). Let the turtles begin! — Chuck

* Recommended age for the game itself. Creating an online account requires age 16+ — younger players can play offline, or online with an account created and managed by a parent.