Plašil, aged 18, was signed by Monaco in 2000 but in his first two-year spell he could only pick 8 matches as a starter and was subsequently loaned to Ligue 2 club Créteil. After a decent performance, Plašil returned to Monaco at the start of the 2003 season and for the next four years he was a regular in the starting team and even enjoyed his finest moment in the 2003–04 season when the principality side reached the final of the UEFA Champions League. In that season, Plašil contributed a goal to Monaco's record-breaking 8–3 defeat of Deportivo.
On 15 June in the Czech Republic's final Group A match against Turkey, Plašil scored in the 62nd minute to give his team a 2–0 lead and apparently send them into a quarter-final meeting with Croatia. However Turkey came back and scored three goals in the last fifteen minutes and, despite Jan Koller getting Turkish goalkeeper Volkan Demirel sent off in second half injury time, Turkey advanced instead.