Leicestershire rider Sarah Cohen saw off a powerful international challenge to land the Grantham Cup CIC three-star title.
The Belton Horse Trials' showpiece class was led overnight by Laura Collett and Rayef, top-10 finishers on their Badminton debut last year, after they delivered an impressive dressage score of 39 penalties.
Collett still held top spot after showjumping, despite collecting four faults, but there was no way back when they had a run-out during the cross-country phase.
The cross-country optimum time of six minutes and 13 seconds proved a demanding test, but 38-year-old Cohen and Treason collected just six time faults to finish on a score of 53.7. The combination had been 15th after dressage and then eighth following showjumping, but as other contenders fell away, so their score proved sufficient.
Australian Paul Tapner guided his 2010 Badminton winner Inonothing to second spot, prompting a jubilant reaction from the Swindon-based eventer.
Inonothing was injured during the World Equestrian Games in Kentucky almost 18 months ago, suffering a chipped bone after hitting a cross-country fence, which caused concern about his future competitive prospects.
But Tapner slowly nurtured him back to the top level, and a superb cross-country round today, when the combination collected just 5.6 time penalties, augured well less than five weeks before this year's Badminton.
Ireland's Sarah Ennis (Sugar Brown Babe) was third, ahead of Italian Vittoria Panizzon (Borough Penny Z), with Sweden's Ludwig Svennerstal (Shamwari 4) fifth and Collett (Noble Bestman) sixth.
Irishman Mark Kyle and Wiltshire-based New Zealander Andrew Nicholson, meanwhile, won the day's two advanced classes in the Lincolnshire sunshine. Kyle, riding Coolio, gained an emphatic victory in section N, finishing 9.3 penalties clear of runner-up Izzy Taylor, who was aboard Thistledown Poposki.
Nicholson, world individual bronze medallist in 2010, steered the experienced Avebury home in section O, with Sarah Stretton and Skip On II second, with former Burghley winning combination Caroline Powell and Lenamore third.