The home side got off to the best possible start with an early try from flanker Hector Bright, converted by scrum half Matt Shepherd.
The Saints failed to capitalise on some good chances and it was the visitors who grabbed the next score through number 8 Brandon Moore to make it 7-5. Flanker Ryan Smale went over with the clock almost on 40 to give Barnstaple a narrow 7-10 lead at the break.
With the heavens opening, the visitors looked to their big pack to take control in a dominant second half display that delivered three tries. A penalty try set the tone and that was quickly followed by another seven points through Lock Arthur Marks, converted by Jordan Petherbridge. A second from Brandon Moore took the score to an unassailable 7-29.
The Saints never gave up and finished the game with a following move that saw centre George Tregilgas crash over for a converted try to make the final score 14-29.
The Saints make the long trek up to the Forest of Dean next week, when they will seek their first points of the season away at Lydney (Saturday 14 September 3pm KO).