After scoring an impressive 37 goals in 30 games with Barça's Cadete A, being the star player of Barça's best Cadete side ever, which didn't lose a single game and won the league, Copa Catalunya and the Spanish Championship, he was promoted to Juvenil B. It would be the beginning of a rollercoaster of a season for him, playing in 6 different teams overall.
He made his debut against Feyenoord in the U-17 Club World Cup final, assisting Songo'o's equalizer, and later winning the game 3-1. Shortly after, he won the José Luis Ruiz Casado Memorial 6-0 against Betis 6-0 with a brace and assist from him, and then the J. Gil Alonso Final against Espanyol 2-1 with a goal from him. After seeing those performances, he was quickly promoted to Juvenil A.
With Juvenil A, he made his debut against Hércules with a goal and won 3-0. He had an ankle injury in early October but quickly recovered from it. Afterward, he got his first poker of the season against Gimnàstic in Barça's 7-0 win. A bit over 2 weeks later, he scored a hat-trick against Granollers, and Rijkaard was made aware of his talent, and so, he was selected for a friendly against Porto.
November 16th, 2004, that was the day of his unofficial debut in Barcelona against Porto after being subbed on 74 minutes in. He had 2 good scoring chances but he unfortunately couldn't convert. Still, he made a good performance despite Barça losing 2-0.
Not long after, he made his debut in Barcelona C against Europa, winning 2-1. And before the end of the year, he won the Trofeo Ciudad de Nerja with Juvenil A, making 2 assists in Barça's 2-0 win in the semi-final against Málaga then 2 goals against Real Betis in the final, winning 3-2.
Entering 2004, after scoring a hat-trick against Gramenet to win Barcelona C the game 3-2, and some other good performances with the C team, he played once against with Barça's senior team, this time against Shakhtar Donetsk, entering in the second half.
Not long after, on March 6th, he made his debut with Barcelona B against Mataró, winning the game 1-0 though getting a yellow card. He made a few other performances with the B team before returning to Juvenil A and bagging a hat-trick against Vilobi in a 5-1 victory.
In the final games of the season with Barcelona, he returned to Juvenil B after getting injured over a week, and he ended up winning the league title with them on May 8th. His last games were with Juvenil A in the Copa del Rey Junior, scoring a poker against Sevilla in the R16, helping with a goal and an assist in Barça's 4-1 aggregate victory over Real Sociedad in the QF, then finally getting eliminated by Osasuna due to away goals, scoring a brace in the first leg.
After seeing those performances from him (40 goals and at least 6 assists in 46 games), and rejecting Spain's offer to play for them, he was called up for the U-20 Argentinian national team. He made his debut on June 29th against Paraguay, scoring an amazing solo goal starting halfway the pitch and giving 2 assists as well in Argentina's 8-0 victory. A few days later, he played against Uruguay in his last game of the season and made a brace as well as giving an assist in Argentina's 4-1 victory.
Overall, it was an amazing experience, starting at Juvenil B and ending up in 2 friendly games with the senior team and breaking havoc all the way up to Barcelona B, even getting called up for the U-20 national team, so it's safe to assume this was Messi's breakthrough season in terms of talent before officially making his debut in the senior team.
