Act I

Seascape begins on a beach where an older couple, Nancy and Charlie, have just finished their picnic lunch. As Nancy tidies up, the sound of a low-flying jet fills the stage. Charlie predicts that one day a jet will crash into a dune.

Nancy expresses her wish to stay by the sea forever, as she adores everything about it and dreams of traveling from one beach to another. However, Charlie dismisses her aspirations, showing no interest in doing anything or going anywhere.

Nancy becomes upset with Charlie's attitude, reminding him that life is short. She even threatens to embark on adventures by herself. This prompts a change in Charlie's demeanor, causing Nancy to backtrack slightly on her plan, content to savor the present moment.

After another jet roars past, Nancy recalls Charlie's childhood dream of living underwater. Charlie shares how he used to sink to the bottom of a pool or lake and sit there until he needed air. Nancy urges him to reconnect with his youth by doing it again, but Charlie, feeling embarrassed, refuses.

Shifting the conversation to their sex life, Nancy reveals a time in their marriage when she considered divorce, suspecting Charlie of having an affair due to the tension between them. Charlie denies this, and Nancy accepts his explanation.

Once more, Nancy encourages Charlie to submerge himself underwater and demonstrate how he did it. Charlie declines again and shifts the focus to her, praising her as a good wife. Nancy reciprocates, listing numerous ways he was a good husband. Despite this, she feels bitter, perceiving their "good life" as limited. Charlie feels hurt by her perspective, leading to an argument. Nancy remains frustrated that Charlie only wants to rest while she desires new experiences.

During a pause in their heated exchange, Leslie, a human-sized male lizard, peeks at them. Nancy tries to involve Charlie in writing postcards, but he refuses. Leslie returns with his female mate, Sarah, and Nancy notices the lizards, intrigued, while Charlie feels frightened.

Charlie demands that Nancy find something to defend them with, but she only finds a small, flimsy stick, which annoys him. Despite her continued interest in the lizards, Nancy becomes apprehensive when Leslie clears his throat, fearing they might be dangerous.

As Leslie waves his large stick, Nancy and Charlie confess their love for each other, with Nancy doing so more reluctantly. When Leslie and Sarah advance, another jet scares them away. Charlie attempts to blame the incident on bad liver paste sandwiches, suggesting they are dead from food poisoning, but Nancy scoffs at the idea.

Nancy is delighted when Leslie and Sarah come back. To ensure their safety, Nancy thinks they should demonstrate submission by lying on their backs with their arms and legs raised, similar to how a dog might do. Charlie adopts this position, albeit very hesitantly.