Ransome used to sail to Hamford Water, an area of tidal salt marshes and low-lying islands, in his yacht Nancy Blackett.

The Swallows intend to sail in the Goblin (as featured in We Didn't Mean to Go to Sea) to Hamford Water and camp with their father, but he is called away on naval business.

They spot somebody escaping in a dinghy, and they track the person to his lair; it turns out that the footprints had been left by a local boy named Donald, whom they nickname the "Mastodon", wearing "splatchers" (mud-shoes), and he left the totem pole because he mistook them for the Eels, another family who camp in the area regularly.

For a surprise, Father has arranged for the Amazons to come down from the Lake District and join them with another dinghy, named the Firefly.

The "sacrifice" is Bridget, who very nearly does not arrive at her party, as a result of being trapped in the middle of a ford by a rising tide with Titty and Roger.