The film opens with Kevin Spacey playing a middle-aged Bobby Darrin in a fictional setting--making the movie about his life--and laughing off how he's a bit old for the part. Then Spacey/Darrin has an exchange with the young actor who is playing Bobby Darin as a boy about how the story should start, and the boy walks out onto the "set" of Harlem in the early 40's, which is the beginning of the biography proper. Viewers see how Wally Robert Cassotto's (Darin's birth name) childhood battle with rheumatic fever (which the doctor believed would kill him) provided him with the foundation to pursue a singing career. Singing was his mother's former profession and she teaches Bobby all the old standards and singer's moves as a way to help him beat his illness. Most of all, she imparts to him the joy that music brings her.
Spacey then takes over as Wally, the young man who vows to be bigger than Sinatra, accompanied with his brother-in-law as a first booking agent. Scenes quickly pass from his first tiny venues to his changing his name based on the words off a sign, to his breakthrough hits, "Splish Splash" and "Dream Lover." His mother lives to see him on the Ed Sullvan show, but dies shortly after.
As a successful entertainer, Darrin begins to make movies and when he makes one with Sandra Dee, falls in love with her and convinces her to defy her controlling mother and marry him. The film goes on to cover how, despite their obvious love for each other, their marriage struggles with Dee's alcoholism and Darrin's drive to be bigger and better, and to Darrin's "sister" Nina's revelation that SHE is his real mother (she had him out of wedlock and didn't know which guy she dated was the father)and that his "mother" was really his grandmother. From there, Darrin spirals out of control for a while as he soul-searches for who he really is. He stops performing and isolates himself from nearly everyone and begins brooding over the state of the Vietnam war. He composes political songs and eventually stages a come back due to his unfailing savvy as a performer. But the strain on his health, which had never been strong due to the rheumatic fever he had, finally takes his life. But as the movie about his life ends, Spacey/Darrin stages a final performance which illustrates how a legend like Bobby Darrin continues to live on. It's one of several musical numbers in "Beyond the Sea" that showcases the unseen stars of the film, the unforgettable songs Darrin created.