I’m the only one at this watch party that’s never seen this movie, so I wrote down what I THINK happens based on what I’ve picked up in marketing/pop culture:

One night in the 70s Mia (Meryl Streep) bones three different dudes in the same night (or it’s one guy at a masquerade party?). Cut to 30 years later, she’s a ‘mamma’ and her daughter “the Dancing Queen” is getting married, and she invites those three guys to Greece to figure out which one is the dad. (Colin Firth, Pierce Brosnan, and Bill Nighy?). Meanwhile Meryl Streep runs a local bakery and an Andy Garcia-type is like “if you don’t pay in 72 hours the bank is taking your bakery!”). So the daughter dances with all three dudes to different ABBA songs around Greece, but can tell neither is her dad by their weak moves. 
Day of wedding, Andy Garcia shows up and is like “gimme my money!”, and he has some similar catchphrase as the daughter. Meryl remembers now, his name was Fernando! They dance to that song and they have the same moves! So Andy Garcia’s like “keep the bakery, I quit the bank!” Everyone dances to dancing queen, credits.
Mid-Credits scene: Andy gets down on one knee abs proposes to Mamma Mia. Meryl winks to camera and says “…here I go again!” Also ABBA cameos as themselves as the wedding band.

No spoiler tag cuz, uhh, I got some stuff wrong