It was the second appearance in as many weeks for Mollie and a surprise debut in the dance off for Aston, one of the favourites to win.
On Saturday, Mollie and her pro partner AJ Pritchard danced the Foxtrot to Call Me Irresponsible by Connie Francis to score 27 points from the judges.
Aston and his pro dancer Janette Manrara had performed the Viennese Waltz to Who's Loving You by The Jackson 5, scoring 25 marks from the judges.
Mollie placed middle of the leaderboard whereas Aston, who had been top in past weeks, slumped to second from bottom in his worst result yet.
As usual, the results from the public vote were added to the judges' scores to decide the bottom two in the dance off tonight.
First to be announced as in the dance off were Aston and Janette.
"We're all shocked," host Tess Daly remarked.
"I feel like I've let Janette down, we had fun on Saturday night and we'll try and have fun again now," Aston said.
Before the pair danced again, Strictly judge Shirley Baallas advised: "You need to bring your performing skills, just because it's a ballroom dance doesn't mean it's devoid of any energy. Just go for it, do the very best you can."
After also being announced as in the dance off, Mollie reacted: "We're gutted, we just need to try and do better in the dance off."
Bruno Tonioli told her: "Saturday night's dance was very good, don't overthink it now. Go out there and enjoy and do your best."
Tonight's Strictly Come Dancing 2017 results
Both Aston and Mollie then danced again for the judges, who had to decide which couple should stay in the competition.
Craig Revel Horwood and Shirley Ballas both voted to save Mollie while Darcey Bussell and Bruno Tonioli saved Aston. As head judge, Shirley had the casting vote and so Mollie was saved and Aston went home.
Strictly Come Dancing 2017 is back next Saturday and Sunday night, live on BBC One.