Sky Bet League Two leaders Plymouth rallied from a goal behind to draw 1-1 with promotion rivals Leyton Orient.Jay Simpson's sensational 18-yard volley fired the O's into a fourth-minute lead.Top scorer Simpson took his League Two tally to 14 goals with his thumping right-foot strike which gave on-loan Brighton keeper Christian Walton no chance on his second start for Plymouth.Argyle skipper and central defender Curtis Nelson levelled in the 81st minute after O's keeper Alex Cisak made a brilliant point-blank save to deny Luke Croll.Cornish keeper Walton - who began his career at Plymouth - was equal to Jean-Yves M'Voto's goal-bound header from a 15th-minute corner as Orient got off to a flying start.Argyle centre forward Jake Jervis missed a golden opportunity to level on 21 minutes when he side-footed across the face of an open goal after being teed up by winger Craig Tanner.Plymouth sub Josh Simpson fired over in the 38th minute after being put in down the right by target man Jervis.Walton then made a superb blocking save to deny O's striker John Marquis on 59 minutes before Nelson's late equaliser.