Greta Gerwig
One question: why isn't it called How I Met Your Father?Reuters

Greta Gerwig has been cast in the lead role of How I Met Your Mother spin-off, How I Met Your Dad.

The Frances Ha and No Strings Attached actress will play Sally, described as a "female Peter Pan" character whose marriage is failing after less than a year.

Introducing an entirely new set of characters from the soon-to-end How I Met Your Mother, the new show for CBS will however be led by the same team of Carter Bays and Craig Thomas. They'll be joined by Modern Family writer Emily Spivey, who will give the show's writing a female perspective.

Structured in the same way as the departing sitcom, How I Met Your Dad will be narrated by Gerwig's character at a later stage of her life, as she tells her children the titular story.

Characters from the show are expected to be introduced in How I Met Your Mother's final, hour-long episode, set to air in the US on 31 March.

It's unclear whether Gerwig will also narrate the show. Bob Saget narrated the previous series as Ted, who was played on screen by Josh Radnor.