Despite its title, The Forever Whale isn't primarily about whales; it's about memories and shared connections. In particular, it's about the close bond between the story's protagonist, Hannah, and her grandfather, whose memory is gradually fading due to Alzheimer's.

Hannah's not prepared to lose her grandfather, and so she comes up with an unusual idea. She's going to make a film that will remind him of his life. Hannah finds out that people with Alzheimer's often remember things from long ago, and so she hopes that by dredging up some important memories, she will give her grandfather the opportunity to get in touch with a past that is fast slipping away from his mind.

While she's making her film, Hannah spots a large whale and captures him on camera. In doing so, she hopes to be able to show him to her grandfather later on, hoping that he will remember the story about a whale that he was going to tell her before he started losing his memory.

The whale is forever in that it is part of a memory that will never truly die, even if Hannah's grandfather loses his memory completely. That's because it is connected to the memories of Hannah's own childhood, which in due course will be passed on to successive generations.