According to one account, however, there was some doubt about Plenipotentiary's paternity: Thomas Thornhill's son-in law claimed that Harriet had actually been covered by a stallion named Merlin.

Plenipotentiary was unraced as a two-year-old but was not unknown to the racing world as he appeared in the betting for the 1834 Derby in October 1833 when he was offered by bookmaker at odds of 30/1 and quickly backed down to 15/1.

[6] At Epsom, on 29 May Plenipotentiary started 9/4 favourite for the Derby in a field of twenty-two runners, with Shillelagh, ridden by Sam Chifney being his main rival in the betting at 3/1.

His first appearance in front of the crowds, however, led to alarm, as he seemed barely able to gallop and was described as looking "more like a pig than a racehorse", causing his supporters to start "hedging" their bets by wagering on other runners.

[9] In the race, Plenipotentiary ran very poorly: he struggled to match the pace of the other runners and despite being ridden hard by Conolly he dropped away to finish tenth behind the 40/1 outsider Touchstone.

[10] Others noted that the normally lively and energetic Plenipotentiary seemed sluggish and unsteady before the race[2] and suggested that he had been drugged by bookmakers or other "scoundrels" who stood to lose a great deal of money if the colt won.

The pace was very strong as his opponents attempted to "choke off" the favourite, but Plenipotentiary won easily, beating Nonsense by a length with Shillelagh among the beaten horses.

His next race provoked great interest as he ran over a sprint distance for the first time, being matched against the 2000 Guineas winner Clearwell in a Subscription Plate over five furlongs.

Although his racing career had ended more than fifty years earlier, Plenipotentiary was ranked eighteenth, having been placed in the top ten by fifteen of the voters.

[13] Sam Chifney was reported to have said that the Derby runner-up Shillelagh was a better horse than his 1830 winner Priam but was a stone (14 pounds) inferior to Plenipotentiary.

[7] Baily's Magazine when comparing great horses expressed the view that the field beaten by Plenipotentiary at Epsom was one of the strongest in the history of the race.

He sired two Classic winners in Potentia (1000 Guineas) and Poison (Epsom Oaks), while the best of his colts was probably The Era, who won a Northumberland Plate and defeated Cotherstone in a private trial.