This Is Us [1] knows how to save the best for last. During the season four finale on Tuesday, there were quite a few surprises [2], twists, and of course, emotional moments. While Dr. K's rendition of "Blue Skies" made us sob buckets, another song seems to foreshadow a big plot point on the show.

During Jack Jr.'s first birthday party, Billy Joel's "Summer, Highland Falls" plays in the background. As Jack Jr. eats his three-layer cake (to represent the Big Three), we also see flashbacks of Kate, Kevin, and Randall celebrating their first birthday in the scene.

Though the melody of the song is generally uplifting, the lyrics are much more profound. In fact, in an interview with Howard Stern in 2010, Joel said, [3] "The lyric is about manic depression. And I wanted music to reflect that. Generally, I'm up, I'm down. I don't think I'm manic depressive. I just know a mood won't last the rest of my life." The lyrics also reflect about a relationship that is on the rocks as Joel sings, "So we'll argue and we'll compromise and realise that nothing's ever changed. For all our mutual experience, our separate conclusions are the same."

Not only does the song perfectly capture the highs and lows of life featured throughout the episode, but it could be a metaphor for Kate and Toby's relationship. As the couple contemplate adopting a child in the episode, Kate reminds Toby that their marriage almost fell apart when Jack Jr. was born and she is uncertain if they could handle having another child. Seeing that the flash-forward reveals that Jack Jr. has a sister, it's possible that what Kate said could come true. Maybe Toby isn't wearing a wedding ring in the flash-forward [4], because they're no longer married.