The series follows major developmental turning points across its nine seasons, leading to significant character transformations and twists:
- Season 1: Ted meets Robin at MacLaren's and pursues her with the blue French horn. Lily panics about marriage, breaks her engagement with Marshall, and moves to San Francisco.
- Season 2: Marshall and Lily reconcile and marry. Ted and Robin date for a year, breaking up when their life goals clash (Robin wants travel/career, Ted wants family). The Slap Bet begins.
- Season 3: Ted dates dermatologist Stella Zinman. Barney secretly falls in love with Robin.
- Season 4: Stella leaves Ted at the altar for her ex Tony. Ted transitions from corporate architecture to university teaching.
- Season 5: Barney and Robin date, but break up after recognizing their mutual toxicity. Ted purchases the rundown house he will eventually raise his family in.
- Season 6: Marshall’s father suddenly passes away. Ted designs the GNB headquarters, leading to conflict and romance with activist Zoey Pierson.
- Season 7: Marshall and Lily have their first son, Marvin. Robin learns she cannot have children. Barney courts stripper Quinn, but their lack of trust dissolves their engagement.
- Season 8: Barney executes 'The Robin' in his Playbook to propose to Robin. Ted struggles with watching Robin marry his friend. The season ends with Tracy buying a train ticket to Farhampton.
- Season 9 & Climax: Barney and Robin marry. Ted meets Tracy at the Farhampton train station under the yellow umbrella.
- The Twist & Finale Epilogue: Three years into marriage, Barney and Robin divorce due to Robin’s career travel. Barney has a baby daughter who changes his life. Ted and Tracy spend ten years together and have two kids (Penny and Luke) before Tracy falls terminally ill and passes away around 2024. In 2030, his kids recognize that the entire story was about Aunt Robin and urge him to ask her out; the series closes as Ted shows up beneath Robin's window holding the blue French horn.