The story begins with Ellie’s pet goldfish that appeared to live for several years. She later learns that it died within months, but Lissa replaced it and continued to replace the fish as they died. Ellie’s belief in a long-lived goldfish collapses. Then her grandfather shows up as a rejuvenated teenager, and his achievement enthralls her until she realizes eternal youth might not be such a good thing.

Each chapter in the book begins with an illustration of goldfish. Chapter 1 contains one fish, Chapter 2 has two, and so on up to Chapters 14, 15, and 16, when there are 14 goldfish and Ellie spends a lot of time with Melvin. After that, each chapter has one fewer goldfish until, in the final chapter, number 29, there’s only one fish left. The fish add up as Ellie begins to learn her grandfather’s enthusiasm for science and discovery; then the fish diminish in number with Ellie’s fading respect for Melvin’s rejuvenation chemical.

Nonetheless, Ellie learns a great deal from Melvin, including the direction her life should take. She calls him her “fourteenth goldfish” (185) because he’s the one who managed to outlive expectations and because his experience teaches her the lesson she couldn’t learn from her pet goldfish: Life persists for only so long, and it’s best to live it fully while it lasts.