Liverpool signed Plessis from Lyon on transfer deadline day in August 2007 after he impressed with the French u-19 side during Euro 2007 in Austria. Holding midfielder Plessis left Lyon claiming that he didn't get any chances to prove himself at the French club for the last two seasons and in order to progress he felt it was best to leave. On securing his signature, Rafa Benítez said: "He is a good player, big and strong, and we're sure he'll do well for us. He's only a young boy with plenty of time to work on his game but it depends on his progress how quickly he can force his way into the first team."
The Frenchman was given the No. 47 shirt for the 2007/08 season and was a surprise choice to start the League match against Arsenal at the Emirates which was sandwiched between the two Champions League games against the same opponents in April 2008. He played the full 90 minutes giving an assured and composed display, as he did three weeks later in another away match at Birmingham City. Plessis made two starts at the beginning of the 2008/09 season, but was replaced at half-time against Sunderland with regular first-choice Xabi Alonso, who had been struggling with injury. As expected Plessis got his chance in the League Cup with other first team irregulars in September and December 2008. Although named in seven matchday squads in 2009/10, Plessis only actually figured once, in the League Cup fourth round tie away to Arsenal at the end of October, the same venue where he had made his senior debut two and a half years earlier. This turned out to be his last game for Liverpool.
At the end of August 2010 Plessis moved to Panathinaikos in Greece for an undisclosed fee. Shortly before Christmas 2011, Plessis was signed on loan by Doncaster Rovers for the remainder of the 2011/12 English season. However, clearance for him to play in England again did not come through until the middle of January and oddly enough he was not selected for a single Doncaster first-team fixture as Rovers were relegated from the Championship. Plessis signed for second division French side Arles-Avignon in July 2012.
Plessis appeared in thirty-two Ligue 2 matches in 2012/13 plus four in the French League Cup. His club, Arles-Avignon, finished in what sounds like a comfortable eleventh position with forty-dix points. But that was only six points above the third relegation spot (18th) and a massive twenty-three points behind the club that was promoted in third place, Nantes. In January 2014 he joined Lausanne in Switzerland on a six month deal then returned to France in the summer, joining Ligue 2 side Chateauroux. After a season there where he made thirty league appearances he left for Portugal, signing for Maritimo, for whom he appeared in the 2016 Portuguese Cup final, which was lost 6-2 against Benfica.
After leaving Maritimo, Plessis was without a club until March 2017 when he joined Orebro in Sweden for their summer season. In February 2018 he moved to the French island territory of Reunion where he has an interest in real estate, joining local side SS Capricorne.