AUGUSTA — After his team played Boothbay twice during the regular season, Monmouth Academy Coach Scott Wing had a good read on the Seahawks during their Class C South girls’ basketball semifinal Thursday at the Augusta Civic Center.

Wing knew which players his team needed to stop, and his group followed the game plan to the letter as the third-seeded Mustangs beat the second-seeded Seahawks, 39-30.

“Today was just a picture-perfect execution of our defensive game plan,” Wing said. “For a bunch of 5-6, 5-7 girls to take away their inside game was pretty awesome, but they paid attention in practice and executed everything we wanted to do.”

The Mustangs (17-3) advanced to the regional final for the first time since 2008. They will play Old Orchard Beach at 7:05 p.m. Saturday in Augusta.

Boothbay, the defending regional champion, finished 18-2.

The key for Monmouth was shutting down Boothbay’s inside game, featuring 6-1 sophomore Faith Blethen and 5-11 junior Page Brown.

“We definitely slowed offensively,” said Boothbay Coach Tanner Grover. “I don’t know if it was our lack of execution, Monmouth’s defense or unfriendly rims, but we definitely had a poor offensive night, however you want to spin it.”

Against the Mustangs’ sagging man-to-man defense, the Seahawks sank only 15 of 56 shots (27 percent).

“They played off of our non-shooters and focused on our scorers inside,” Grover said.

“Monmouth’s a good team. The had a good (defensive) scheme and they stuck with it.”

The Mustangs shot slightly better, sinking 37 percent of their shots (10 of 27). Much of their scoring edge came from the foul line, where they sank 16 of 20 free throws (80 percent).

“As a team, we’re about 68, 69 percent (shooting free throws), which is pretty good when you consider all of the kids who shot free throws during the year,” Wing said.

Monmouth held a 19-14 lead at halftime.

Tia Day, a junior guard who scored 21 points, hit back-to-back 3-pointers to help the Mustangs open a 13-8 lead less than two minutes into the second quarter. Two free throws from Abby Ferland and another outside shot by Day stretched the lead to 19-12.

Boothbay rallied in the third quarter, and the Seahawks carried a 25-23 lead into the fourth. With 4:12 remaining, the Seahawks still held a 27-26 advantage, but they were unable to sustain their momentum.

The Mustangs went into the bonus with 3:39 left and took advantage at the free-throw line.

Brown led Boothbay with 12 points.

During the regular season, the teams split two games. On Dec. 22, the Seahawks won 44-37 in Boothbay Harbor. On Jan. 21, the Mustangs downed the Seahawks 49-36 in Monmouth.