Back to the Future Part II

On October 26, 1985, Dr. Emmett Brown arrives in his flying time machine and persuades Marty McFly and his girlfriend, Jennifer Parker, to come back to the future with him to help their future children. Biff Tannen witnesses their departure. They arrive on October 21, 2015, where Doc electronically knocks out Jennifer and leaves her asleep in an alley, explaining that she should not have too much knowledge of future events. He has Marty pose as his own son to refuse an offer to participate in a robbery with Biff's grandson Griff, thus saving both of Marty's children from prison.
Marty switches places with Marty, Jr., and refuses Griff's offer, but Griff goads Marty into a fight. Griff and his gang are arrested, saving Marty's future children. Before rejoining Doc, Marty purchases an almanac containing the results of major sporting events from 1950 to 2000. Doc discovers it and warns Marty about attempting to profit from time travel, but before Doc can adequately dispose of it, they are interrupted by the police, who have found Jennifer incapacitated and are taking her to her 2015 home. They pursue, as does Biff, who has overheard their conversation and picked up the almanac which Doc discarded.
Jennifer wakes up in her 2015 home and hides from the McFly family. She overhears that her future self's life with Marty is not what she expected, due to his involvement in an automobile accident. She witnesses Marty being goaded by his co-worker Douglas J. Needles into a shady business deal, which leads to Marty's firing. Attempting to escape the house, Jennifer encounters her 2015 self and they both faint. While Marty and Doc attend to her, Biff steals the time machine and uses it to travel back to 1955 and give the almanac to his younger self to get rich betting, then returns to 2015. Marty, Doc, and an unconscious Jennifer return to 1985, unaware of Biff's actions.
The 1985 to which they return has changed dramatically: Biff has become wealthy and corrupt, and has changed Hill Valley into a chaotic dystopia. Marty's father, George, was killed in 1973, and Biff has forced Marty's mother, Lorraine, to marry him. Doc has been committed to an insane asylum. Marty and Doc decode evidence that 2015 Biff took the time machine to change 1985, and Marty learns from 1985 Biff that he got the almanac on November 12, 1955. Biff attempts to kill Marty, but Marty flees and returns to 1955 with Doc.
Marty secretly follows the 1955 Biff and watches him receive the almanac from his 2015 self. Marty then follows him to the high school's dance, being careful to avoid interrupting the events from his previous visit. Marty and the 1955 Biff steal the almanac back and forth, but Marty and Doc retrieve it and leave Biff to crash into a manure truck. Marty burns the almanac, reversing Biff's changes to the timeline, as Doc hovers above in the time machine. Before they can return to 1985, the machine is struck by lightning and disappears. A Western Union courier immediately arrives and delivers a letter to Marty; it is from Doc, who explains that he was transported back to 1885. Marty races back into town to find the 1955 Doc, who had just helped the original Marty return to 1985. Shocked by Marty's sudden reappearance, Doc faints.

Question: Who does Marty switch places with in the beginning?
Answer: Marty Jr