The music legend, 70, appeared on The One Show and revealed that the box office smash-in-waiting is flashback-heavy.
“It’s more of a prequel really, because it’s about everything that happened before they ended up on Kalokairi island,” he said.
“It’s Donna when she was young, Meryl Streep when she was young, and what happened when she met the three dads.
“As well as what happened after the previous film finishes. So it’s a prequel and a sequel.”
Asked if Pierce Brosnan will give another attention-grabbing vocal turn, he confirmed: “Absolutely. Because he’s a good singer.”
Mamma Mia: Here We Go Again! will star Lily James as the younger Donna (Meryl Streep), Jessica Keenan Wynn as the younger Tanya (Christine Baranski) and Alexa Davies as the younger Rosie (Julie Walters).
The likes of Colin Firth, Brosnan and Stellan Skarsgard will played in their younger incarnations by Hugh Skinner, Jeremy Irvine and Josh Dylan respectively.
Amanda Seyfried and Dominic Cooper are also back.
Cooper recently enthused to Digital Spy: “I thought it was going to resurface much more quickly than this, but what’s great is that they’ve waited. They haven’t just immediately thrown another one out there on the back of the success of the last one.
“They’ve waited for a script to come along that makes sense and is fun and is gonna be as exciting for fans of the original, and that’s really important.”
Mamma Mia! Here We Go Again is out on July 27, 2018.