Backup quarterback Darell Garretson threw for 321 yards and three touchdowns, leading Utah State to a 35-20 upset of No. 18 Brigham Young on Friday night.

BYU quarterback Taysom Hill, a Heisman Trophy hopeful, broke his left leg late in the first half and is likely done for the season. Without him, the Cougars (4-1) never recovered from a 14-point halftime deficit on a night when former star Jim McMahon had his No. 9 jersey retired.

Garretson, a sophomore who took over for injured Chuckie Keeton in the second half of a Week 3 win over Wake Forest, also ran for a score. He completed 14 of 17 passes for 257 yards and three TDs in the first half, propelling Utah State to a 28-14 lead.

Hunter Sharp had five catches for 173 yards and a touchdown for the Aggies (3-2), who were 20-point underdogs. Devonte Robinson added six catches for 98 yards and two TDs.

Sophomore linebacker Nick Vigil, who played on both sides of the ball for the first time in his career, carried 16 times for 57 yards and one TD. Vigil also led Utah State with nine tackles.

The Aggies racked up 457 total yards to BYU's 425. Utah State also had nearly a 16-minute advantage in time of possession. The Aggies forced BYU into four turnovers.

For BYU, Jamaal Williams rushed for 99 yards. Mitch Matthews had eight catches for 117 yards and one TD. Jordan Leslie caught four passes for 135 yards.

Hill began the game in familiar style when he went deep down the right side to Leslie for 53 yards on the first play. Four plays later, Hill scrambled left from the 11-yard line and hurdled a defender at the goal line to put the Cougars up 7-0 only 1:16 into the game.

After fumbling the ball away on its first drive, Utah State came back to tie it at 7 on its next possession. Garretson capped the 10-play, 85-yard drive by connecting with Robinson over the middle on a third down from the 7-yard line.

The Cougars regained the lead at 14-7 when Hill lofted a pass to Matthews in the end zone from 25 yards out.

Utah State responded with another long march, tying the score 14-all when Garretson rolled left and dove across the goal line for a 5-yard TD run.

BYU fumbled on the next play, with the Aggies recovering on the 22-yard line. One play later, Garretson found Robinson wide open in the end zone to put Utah State up 21-14.

Minutes later, Garretson stunned the home crowd when he found Sharp streaking all alone deep down the left side for a 72-yard TD pass right before the break.

Garretson went back to work downfield in the third quarter. He again found Sharp behind BYU's defenders for 56 yards, setting up a 1-yard TD run by Vigil.

All BYU could muster in the second half were two field goals by Trevor Sampson. Reserve quarterback Christian Stewart, taking over for Hill, threw three second-half interceptions.