Relegation-threatened Livorno have sacked Domenico Di Carlo after just 88 days in charge and re-appointed Davide Nicola.
Virgil van Dijk claims he knows nothing of interest from English clubs and is keen to prove himself in the Champions League with Celtic.
The 18-year-old arrives at Craven Cottage on a free transfer and will be part of the Fulham development squad that will compete in the Barclays Under-21 Premier League.
Plumain represented France at Under-16 level and he made 22 appearances for Lens in the French second division last season, scoring one goal.
He has signed a contract with Martin Jol's team until June 30 2016, with the option of extending the deal for another year.