it's set in 2013 (five yrs.later, see description), thus fthis year, 2013, would be five years later thus the setting for a sequel..p very good, by the way. But, though Lily James is playing Meryl Streep as roughly (the current) Amanda Seyfried's age or younger, she's a dead ringer for Amanda herself..
shareAnd adding to that, the flashback story starts '78 or '79, which sets the Mamma Mia! (2008) around 1998. It's off in many ways :)
shareFlashback starts in summer '79. Sophie born 1980 (most likley). She says to Cher she is about 25 years to late to be a grandmother to her, so MM2 must be set in 2005 and the original in 2000.
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