The 49ers got off to an excellent start when Aaron Lynch blocked a punt from Pat O'Donnell and the home side took over on Chicago's eight-yard line. Colin Kaepernick then found Michael Crabtree in the endzone in the flat for a three-yard score.
Jim Harbaugh's men extended their advantage as Kaepernick led his side on a long drive into Bears territory, but they could not add another touchdown and were forced to settle for a 27-yard field goal from Phil Dawson.
The home side thought that they had added another seven points when Frank Gore broke free down the middle and raced into the endzone for the score, but it was called back for a holding penalty. However, Gore did score on the next drive with an eight-yard run for a touchdown.
Chicago were struggling to get anything going on offense until they pulled a score back right before the break, as Jay Cutler lofted the ball into the endzone and Marshall pulled off an impressive one-handed catch for the score.
In the second half, the home side had a long drive as Kaepernick moved them the length of the field, but they were forced to settle for a field goal from the boot of Dawson from 24 yards.
The Bears were inspired by their defensive stand as Cutler responded with a strong drive of their own, which culminated in him finding Marshall in the endzone for a five-yard strike, as he outmuscled Jimmie Ward.
Matters got worse for the 49ers when Kaepernick threw an interception into the hands of Kyle Fuller, who returned the ball to the six-yard line of the home side. Cutler found tight end Martellus Bennett with an accurate fade route to the corner to put his side ahead.
The pressure began to tell on Harbaugh's men as Kaepernick was intercepted for the third time in the game, and second by Fuller, which resulted in another score for Marshall as he beat Ward again for a three-yard score.
San Francisco had one late chance to try to level the scores, but the quarterback's pass into the endzone for Crabtree fell through the receiver's hands, allowing the Bears to kneel out for the win.