Siblings, John, Emily, and Alex are spending the summer at a resort in the Irish village, Howth. They are under the care of their Nurse. John is the oldest, then Emily, and then Alex—who is four-and-a-half.

One afternoon during lunch Alex asks what they will all be playing today. Emily says that he will not be joining them as he is too small. Alex is upset by this and starts to cry. The Nurse defends Alex and says that he is not as small as they think that he is. She explains that Alex walked all the way to the end of the pier by himself the previous day. Alex tells them about his walk.

The other children are not impressed. Alexander begins to cry again. The Nurse says that the children should work together to build a sandcastle. John wants to build the best sandcastle ever built. He begins to leave and beckons for Emily to come help. Alexander struggles to follow. Alexander finally catches up with them only to disappear briefly and then return with an old bucket that he found.

They begin to argue about where to build the castle. Alex puts the bucket on his head and pretends to be a general. Emily tells him that John is always the general. They follow John to where he has claimed a spot to build the castle. Emily and John order Alex around. They tell him to get the things to build the castle. Alex politely gets the shells for the castle and then asks if they need anything else.

They ignore him. Emily has a lollipop. Alexander asks for a lick. Emily ignores him. She then tells him to go away. Alex asks again. This time Emily yells at him and throws sand at him. As he goes to get sand in retaliation, he sees the Nurse in earshot. Instead of retaliating, he begins to scream and cry that he has been blinded. Emily urges him to come back. She is afraid she will get in trouble. John says that she did indeed throw sand at Alex, as he continues to work on the castle. Emily gives Alex the lollipop and he stops crying.

They continue working on the sandcastle. They have Alex go get them water. They cheer him on as he runs back with water in the leaky bucket. They begin to work harmoniously on the castle. John and Emily build a window in the castle. Alex wants to look through it, but they do not answer him. John asks Alexander patronizingly what he thinks of their castle. Alex says “Gosh” repeatedly as he jumps up and down.

Meanwhile, the tide begins to come in and threaten the castle. They begin to pretend to fight the ocean but are disheartened when they realize that the situation is hopeless. Emily says there is one thing they can do. She says she is the queen of the castle and jumps on it—destroying it. She is happy as now the ocean cannot claim it. Her siblings join in. They all jump together onto the castle. Soon this turns into a new game where they all see who can jump the highest.