They dominated the first half of Saturday's third round tie and defended their hearts out in the second to run out 17-10 winners.
Hosts Lutterworth started with the wind at their backs and they used it well as they raced into a two-try lead through Luke Ball and Tom Goldsworthy. Robin Eager kicked both conversions.
After the turn, Towcestrians had the wind in their favour and they made the most of it. Their determination to get back into the game was rewarded as a race for a high kick was won by their winger, who had the simple job of dropping over the line.
They continued to pound the home line as the heavy rain arrived. Try two for the visitors finally came as a fine passing sequence was finished off in the corner. The difficult conversion was missed, leaving the score at 14-10.
Towcester threw everything forward to try to narrow the gap but a master class of tackling and defending kept them at bay. And instead, Lutterworth extended the lead to seven points as Eager kicked a penalty, although this was still not enough to kill off the game. A powerful display, especially by the forwards, was necessary to keep the visitors out until the close.
Lutterworth coach David Johnson hailed "an outstanding performance against the best side we've played this season".
Lutterworth 1st XV won a hard-fought league derby against Old Newtonians 25-18.
The tenacity and commitment of all 30 players led to a penalty-strewn, untidy but nevertheless, enthralling encounter.
The game began at a frenetic pace and the first half developed into a penalty-kicking contest. Lutterworth trailed at the break by six points but fly half Andy Burgess had kept his team in touch with two conversions.
The return, after a long injury-induced absence, of Paul Wilford lifted Lutterworth. Together with Marcus Stevens and Richard Jones, the front row regained the dominance that had been missing for some time, allowing the rest of the pack and especially the back row of Chris Gregory, Matt Randle and Charlie Turner to control the contest. For a second week running, captain Gregory was hugely influential at No 8 with Randle and Turner typically working their socks off on the flanks.
But another old favourite, Paul Riley, who after the break joined Wilford and Jones in the front row, was the catalyst for the team's 19-point second half resurgence. Soon after the re-start they were awarded a penalty try to take the narrowest of leads. Newts hit back with a penalty but then Riley burst over the try line from a five-metre lineout.
The pressure was now constant from the home side and Riley was involved again, taking the ball on from rampaging lock Adam Welsh. Centre Charlie Roberson finished the move in unstoppable style to seal the game.
Newts pressed during the closing minutes but despite having converted six penalties, they failed to cross the home line. With three tries to their credit, Lutterworth's second half display, in testing conditions, made them worthy winners.