 He made his first appearance for Argyle on 26 September, over eight years after he first signed for the club.He was offered a new contract by Plymouth at the end of the 2017–18 season.He was again offered a new contract by Argyle at the end of the 2018–19 season, but turned it down after new manager Ryan Lowe stated he wanted to bring in a new goalkeeper whether Letheren stayed or not.