Short Answer 
Natascha McElhone played Alex Kirkman in ‘Designated Survivor’ for the first two seasons before leaving to focus on her family and pursue a lead role in ‘The First.’ Her departure significantly impacted the show’s storyline and altered character dynamics, highlighting the importance of her role in the series.
Step 1: Natascha McElhone’s Role
Natascha McElhone was known for her role as Alex Kirkman, the wife of the main character, Tom Kirkman, played by Kiefer Sutherland in the TV show ‘Designated Survivor.’ She portrayed this character for the first two seasons of the series, contributing significantly to the storyline during her tenure.
Step 2: Reasons for Leaving
She chose to leave ‘Designated Survivor’ primarily for two reasons: to focus on her family and to take on a new opportunity. Specifically, Natascha aimed to explore a lead role in another show called ‘The First,’ which centers around a human mission to Mars. This decision reflects her priorities in both career growth and family commitments.
Step 3: Impact of Her Departure
The exit of Natascha McElhone from the series marked a significant shift in the show’s storyline. Her character’s departure not only altered the dynamics of the plot but also posed challenges for the main character moving forward. This pivotal moment shaped the series’ direction in subsequent seasons, demonstrating how integral her role was to the overall narrative.