L'Aventure benefitted from a late mistake by One Knight to win the Welsh National at Chepstow.
The pair came to the last together but One Knight fell on landing to present victory to Leighton Aspell's mount.
The winner, fourth in this race last year, used her experience throughout to come in at 14-1.
It was a superb day for trainer Paul Nicholls, who also saddled Heros Collonges in second and third-placed Cornish Rebel.
Aspell was not due to ride in the race but Richard Johnson gave up his booking on One Knight after suffering a crashing fall at Sandown on Monday.
Paddy Brennan moved from L'Aventure to One Knight giving Aspell what turned out to be a lucky ride.
One Knight had led for much of the race and was comfortably ahead with four fences to go before the rest of the field, led by L'Aventure, moved up to challenge.
L'Aventure looked the stronger of the two as the pair approached the last while her stablemates fought out a tremendous battle for the other places.
"That was a brilliant run. L'Aventure was fresh and well and she's done it well," said trainer Paul Nicholls.
"You can see I was a bit frustrated that Paddy (Brennan) didn't stay with her while the other two ran well with big weights."
Earlier, the Alan King-trained My Way de Solzen stayed on well to claim a surprise win in the Long Walk Hurdle.
Robert Thornton's mount (12-1) travelled superbly and was in contention entering the closing stages.
After jumping the last with long-time leader Neptune Collonges, the five-year-old proved stronger on the run-in.
Odds-on favourite Inglis Drever was well out of contention when falling at the second last.
Coral Welsh National
1. L'Aventure (L Aspell) 14-1
2. Heros Collonges (Christian Williams) 25-1
3. Cornish Rebel (R Walsh) 9-2 Jt Fav
4. Crystal d'Ainay (R Thornton) 12-1
18 ran. Dist: dist, nk
Long Walk Hurdle
1. My Way de Solzen (R Thornton) 12-1
2. Neptune Collonges (R Walsh) 6-1
3. Starzaan (T J Murphy) 66-1
8 ran. Dist: 5, 12