Handler Paul Nicholls' two runners lost out in a tight, compelling conclusion to the 20-1 shot, ridden by Paul Moloney.
Tidal Bay, aged almost 13 years, nonetheless proved Nicholls' pre-race sentiments to be spot on by mixing it with the best once again.
Highland Lodge, favourite going in at 13-2, led much of the race before fading on the heavy ground at Chepstow.
"I couldn't believe how well he was going and then Paul gave him a breather and he just ran to the finish," Mountainous trainer Richard Lee told Channel Four Racing.
"He's such a nice horse, I said when we brought him in as a five-year-old he was going to be a Welsh National winner one day."