- [Narrator] Another spy goes undercover.

One with a camera in its shell.

It's arrived at the perfect moment.

The crabs line up in order of size.

They don't all need grand accommodation.

They just need the next size up.

First, they must find their correct place in the queue.

They wait until everyone is assembled.

Then the house swap begins.

The largest leaves his old home and moves into the vacant shell next door.

Then the next size down takes over the shell he left behind, and so it goes on in order of size.

Most are content with their new home, although one seems to have house envy.

He's eyeing up our spy.

He just needs to check the shell for size.

It meets his requirements so he doesn't waste time.

But this home comes with responsibilities.

Ones that involve filming, too.

But our new spy seems to be having second thoughts.

His old shell was a better fit after all.

He'll keep it for a little longer.

Our spy is now the only crab left homeless.