Except in this case, it's real life. That young unknown was Brandon Crawford, who graduated from Foothill High School in 2005 and went on to play at UCLA before getting drafted in 2008.
Crawford described himself as "nervous, but kind of an excited nervous," walking to the plate in Milwaukee, where he hit the game-winning homer.
"I knew I was ready. I was preparing for it my whole life," he said.
That's a pretty accurate summary. Crawford began his career with Tee Ball when he was 4 and went on to play in Little League and school teams before being drafted for college play.
His debut grand slam puts him in an elite league with just six other players, although he's the second Giant to do so: Bobby Bonds hit one his first season in 1968.
"I knew I'd hit it pretty good, but I wasn't sure off the bat. I was just happy I hit it well. I knew we were going to get at least one run in, then saw it take off," Crawford said, sitting in the Giants dugout on a recent Sunday morning.
And while often, a player who knows he has a solid hit -- a Babe Ruth or Hank Aaron, for example -- will wait to see if he cleared the wall, that wasn't the case for Crawford.
"I was running right away," he said, thinking, "If that gets off the fence, I want to be on third."
It might not have been the "shot heard 'round the world," but it was certainly the shot heard 'round Pleasanton. Of course, literally batting 1000 with a run and three RBIs that first hit, the only way for Crawford to go was down. He admits he's been struggling a bit recently.
"It's been up and down," he said. "I've had some other big moments. My first game here I hit an RBI double down the line. I hit a triple against the Nationals. Obviously my numbers are down -- it's been up and down the whole time, really."
"Hitting is one of the most frustrating things in all the sports," he continued. "There aren't too many things you do in sports where you fail seven out of 10 times and are considered a legend."
Crawford grew up watching the Giants. He said his father Michael is a diehard fan, and watching former Giants like fellow shortstop Royce Clayton, Alex Rodriguez, "before he went to the Yankees," and J.T. Snow, it was only natural that he'd want to be on the team.
"Watching them growing up, I definitely wanted to be out there," he said, nodding toward the field. "As I got older, pretty much whoever would draft me, I wanted to play for them."
He described getting drafted by the Giants as "pretty awesome." As a fan turned pro, Crawford has the unique perspective of having been on both sides of the fence.
"It's been really different," he said. "I try not to focus on what's going on around the stadium when I'm playing, where as a fan, you're looking at everything. I'm trying to be as calm as possible out there. If I'm up at bat, I don't hear anything, I'm focused."
But Crawford admitted some things are hard to ignore, like seeing his face on the giant electronic billboard or hearing people rooting for him.
"It was a pretty cool experience, to be a fan and to be chanting other peoples' names for so long and then actually to be here, with the home crowd, having them all loud and cheer my name," he said.
While his friends generally treat him like they did in high school, a recent trip to Pleasanton to see his little sister Eva play softball showed Crawford he has a strong local fan base.
"That was pretty crazy," he said. "There were a lot of people that came out."
As is the case with many so-called overnight successes, Crawford spent years working toward his dream.
"You have to keep working every day. I'm not going to say I did something every day when I was younger, but you have to work on it, more than just going to practice and doing the drills and then going home and not worrying about it. You have to do stuff on your own," he said, explaining that he started going to a hitting coach when he was 13.
These days, he'll typically show up at 2 p.m. for a 7 p.m. game and spend some time in the batting cage before taking batting practice. The training doesn't stop when the season ends, either; off-season, he works out and runs and continues to take batting practice.
Crawford was named UCLA's MVP in 2006 and 2007, and helped the Bruins reach the NCAA Regionals three years straight. He was drafted by the Giants in the fourth round in 2008 and worked his way up. After a brief stint in the Arizona League in August 2008, he split time between the San Jose Giants and Connecticut Defenders (double-A, Eastern League) in 2009. In 2010, he played with the Flying Squirrels based in Richmond, Va., but missed nearly two months of the second half of the season because of a broken wrist.
He was ranked the sixth best prospect in the Giants' organization by Baseball America in 2011 although he has since broken a finger, in the final week of spring training.
Crawford particularly likes that the game is a joint effort.
"You need every part of your team," he said. "We wouldn't be in first place right now without our pitching, and then obviously you need runs to score, so offense is a key part also."
On a personal level, in a city that treats its baseball players like the English do the royal family, Crawford said he's still dealing with the idea of being known by random people on the street.
"It is weird getting recognized even out here around the city," he said. "I still feel like the same person, kind of on a bigger stage. As long as people are just normal to me, I'm going to be the same way back."
Even though he's been on track to becoming a pro for some time, Crawford said he's not really sure when the change was made from just wanting to do it to thinking he could.
"I had always wanted to do this and I never really thought about doing anything else," he said. "I don't know what I'd have been if I wasn't a baseball player."