Wales have only named three specialist second-rows in the Nations Championship squad with Beard and Ben Carter starting against Fiji.
Wales received a blow when Dafydd Jenkins was ruled out of the tour following shoulder surgery.
Beard only linked up with Wales on Sunday following his role for Montpellier in the Top 14 final defeat byToulouse, while Williams played his first game in nine months against Barbarians after recovering from a serious foot injury.
So Tandy has opted for Plumtree to cover lock rather than Williams this weekend.
Wales have used Plumtree primarily as a blind-side flanker or number eight in his 14 internationals.
He has not started a senior game at lock for club or country since he arrived in Llanelli from Auckland before the the 2023 World Cup.
Plumtree’s previous start in the position came while playing for Wellington in the NPC in August 2022, while he also played a handful of games there for New Zealand Under-20s.
Wales chose not to select Gloucester’s Freddie Thomas or Dragons uncapped Ryan Woodman, who have both played at lock and flanker this season, in the Nations Championship squad.
Christ Tshiunza is currently unavailable for Wales selection with Tandy prevented from picking the Sale-bound forward because of the 25-cap rule.
The 24-year-old lock or flanker will switch to Sharks from Prem runners-up Exeter this summer.
It means he will fall foul of the Welsh Rugby Union (WRU) selection policy that requires players that represent clubs outside of Wales to have at least 25 caps with Tshiunza on 15.