The Pilgrims, looking to move further away from the relegation zone, fell behind after 34 minutes when Liam Sercombe opened his account for the season.
But the visitors hit back and levelled through Lennox with 17 minutes to go before Maxime Blanchard saw red late on.
A late strike from substitute Harry Worley earned Oxford a 1-1 draw at Morecambe.
Worley turned home Alfie Potter's right-wing cross from eight yards with two minutes left to cancel out Kevin Ellison's 78th-minute opener.
All three of the league leaders were held to draws. Frontrunners Gillingham played out a 2-2 draw at home to Fleetwood, while Port Vale shared the points with Cheltenham as their match ended 1-1.
Rotherham moved up to fourth with a 1-0 win at AFC Wimbledon, while Bradford dropped to fifth with a 2-2 draw at Southend.
Burton Albion were held at Chesterfield with the two sides consolidating ninth and 10th places, respectively.
Torquay drew 1-1 with Northampton at Plainmoor, while Dagenham and Redbridge beat Barnet 1-0 courtesy of Matthew Saunders’s goal.
York City thumped Bristol Rovers 4-1 at Bootham Crescent, while Aldershot beat Rochdale 4-2 in a pulsating encounter at the Recreation Ground.
Morecambe and Oxford United shared a 1-1 draw at the Globe Arena, while Wycombe clinched a 2-0 away win at Accrington.