Shawn is playing in a police league softball game for the SBPD. Henry is the umpire and calls Shawn out at home plate, thus causing them to burst into a shouting match. There is definitely some leftover tension between Shawn and Henry from his Little League days. Henry ends up throwing Shawn out of the game.
After the game, Shawn is shocked to see Mel Hornsby, the Coach of the Seabirds, walking toward him. Mel tells Shawn that his bench coach, Grady Barrett, suddenly died of a heart attack, but he believes there was foul play involved. Grady's system had amphetamines in it, and Mel knows that Grady would have never put his weak heart at risk by doing drugs. Mel hires Shawn to investigate, and appoints him as hitting coach for the Seabirds as a cover. This excites Shawn, and the fact that his idol Cal is back on the team is the icing on the cake for him. Shawn gets Gus to join the investigation disguised as the team Mascot, forcing him to suit up in an enormous, flea-infested Seabird costume.
Shawn and Gus go to Grady's house. There, Shawn takes a swig out of a sports water bottle sitting on a table in the living room that is, unbeknownst to him, laced with amphetamines. He is left hyper and largely uncontrollable, until the effects wear off. With Henry's help, Shawn figures it out that the sports drink belonged to the Seabird's top player, Izzy Jackson, and not to Grady. Their bottles got switched at hitting practice by accident.
Shawn and Gus go to find Izzy at a bar the players frequent. They determine that Izzy is clean and isn't the one taking amphetamines to boost his performance on the field, as they had originally thought. Then Shawn and Henry put it together that someone who wanted Izzy off the team, and tried to get him busted for drug use by spiking his sports bottle... but who? They run into Cal Eason at the bar, who relays to Shawn and Gus that he would do anything to get himself back into the majors as a first baseman instead of a catcher, as his knees are starting to go. But Izzy, the first baseman and star player, is overshadowing him.
With Henry's help, Shawn puts it together that if Izzy were caught with drugs in his system, he would be suspended, thus creating an opening for Cal to take Izzy's playing time. Shawn approaches Cal during batting practice and accuses him of trying to drug Izzy, but Cal denies it.
Back in the dugout during a game, the General Manager of the team, Neil Stillman, steps in and asks why Izzy isn't playing. Mel says it's because of Izzy's attitude problem. But the GM makes it clear that he wants Izzy playing no matter what. And with that, Izzy is put back in the game.
Shawn watches Izzy at bat. Shawn predicts the next pitch, and is spot on. Shawn predicts the following pitch and, Mel, impressed by Shawn's abilities, tips the third base coach who then signals Izzy what the next pitch will be. As a result, Izzy hits a homerun, and Shawn sees Izzy making eyes at a fellow player's wife who is sitting in the stands cheering for him. Shawn then thinks it must have been this particular player that found out about Izzy sleeping with his wife and tried to get him suspended.
Shawn decides to tell a few of the players on the mound about this during the game. However, Shawn's theory backfires when he learns that the entire team has been sleeping with each other's wives and girlfriends. An enormous brawl ensues.
Mel lectures the entire team for their crazy behavior on the field. The General Manager fires Shawn from the team and replaces him with Wade Boggs, as in the actual baseball legend Wade Boggs.
As Shawn is leaving the locker room, he sees Izzy dead on the floor, whacked on the side of the head with a baseball bat. Mel is the first suspect interrogated at the SBPD, and Shawn is the only one who believes Mel didn't kill Izzy; Henry, Juliet and Lassiter are not convinced. It turns out they found a bag of amphetamines in Mel's belongings. Shawn knows it wasn't Mel and gets Woody to help confirm it. Woody tells them that the person who hit Izzy with a baseball bat struck hard and fast and at a certain angle. Shawn looks at the report, and the bat used to kill Izzy was a size 35.
Now Shawn thinks that it was Cal who killed Izzy because the murder weapon was a size 35 bat and he believes that Cal is the only player on the team who uses that size. Shawn confronts Cal and gets confirmation that it was not Cal, because it turns out that Cal is leaving to go to Oakland to play in the majors again.
Shawn finds a way to continue his investigation in the team's locker room by disguising himself as the mascot. Shawn finds Izzy's contract and "envisions" the words "Bonus pay after time at major league level," and then "Must be traded or promoted after sixty games." From there, Shawn puts it together that it was the General Manager who tried to have Izzy's contract suspended by planting drugs in his system, but his plan backfired when Grady accidentally ingested the drugs instead.
The GM made the mistake of signing Izzy to a $12 million contract, one that he would have to pay in full even if Izzy couldn't hit a curveball, which he discovered, as the season wore on, was indeed the case. When his attempt to get Izzy suspended by flunking a drug test didn't work, the GM desperately tried to trade Izzy to another team before having to shell out the 10 million dollar bonus for being promoted to the big leagues, a bonus that was guaranteed after Izzy played sixty games in the minors. And when there were no takers, the GM decided to kill him and collect the insurance so the team wasn't out the money and he could keep his job.
During the final showdown, the GM finds out that Shawn and Gus are on to him and holds them at gunpoint. Luckily Cal shows up because he had a hunch about the GM being the culprit. Then Wade Boggs shows up, and the GM tries to run away. But before he can escape, Henry knocks the GM out and saves Shawn, Gus, Cal and Wade.