Blackwood Stars, holding second place in Division 5 South-East, turned the game their way when Hafodyrynys, the Division 6 South-East champions ran out of steam at Blackwood's Glan-yr-Afon Park.
Smith, the former Tredegar and Blackwood player, has always been a try-scoring full back, but even by his high standards, the try was a sensation as he ran from deep, beat man after man, kicked ahead and outstripped all the remaining defenders to score.
The Stars were hardly in the game for 30 minutes, yet snatched the lead when centre Rhys Harper intercepted near his own line and scored at the other end for Simon Hughes to convert.
Flanker David Tovey crashed over in reply and Derry Gurmin converted to level it, yet the Stars scored twice more against the run of play to lead 17-7 at the interval.
Wing Kelton Nicholas joined his pack and then the elusive Harper looped to cross again.
After 49 minutes Smith's wonderful effort was converted by Adam Jones and centre Dane Williams also crossed for Jones to convert as Hafodyrynys now led 21-17.
Lock Dan Tucker restored the Stars lead, but Jones put his side back in front with a penalty. Then, Hafodyrynys noticeably tired with a penalty try, another from Tucker and a good effort by flanker Stuart Hayes bringing five-pointers, all converted by Hughes for a 43-24 lead to Blackwood Stars.
However, referee Simon Beard let play flow and Hafodyrynys gained a fourth try from replacement flanker Gethin Foxwell that Jones converted in the final minute.
Full back Smith was named the man of the match.
Blackwood Stars - Tries: R Harper 2, D Tucker 2, K Nicholas, S Hayes, pen try. Cons: S Hughes 4.
Hafodyrynys - Tries: D Tovey, R Smith, D Williams, G Foxwell. Cons: A Jones 3, D Gurmin. Pen: A Jones.