In Dort, Rosa , has met Van Systens , President of the Tulip Society, and failed to prove to him that she, not Boxtel, is the rightful owner of the black tulip. Meanwhile, at Loevenstein, Cornelius has been sentenced to death for attacking Gryphus.

Rosa has frustrated the attempts of Boxtel, alias ' mynheer Jacob,' to steal the bulb of the black tulip, and secretly brought it to flower in her room. Believing that they are alone in the prison, she has taken Cornelius from his cell to show it to him.

Cornelius has been sentenced to life imprisonment in the grim fortress of Loevenstein. Rosa has taken charge of the precious bulbs and promised Cornelius that he will see them again.

Through Boxtel 's treachery, Cornelius has been arrested for being in possession of treasonous letters and taken under guard to The Hague. Boxtel, realising he must have taken the bulbs of the black tulip with him, and determined to have them, has followed.

Cornelius has grown the bulb of a black tulip. De Witt , in prison for treason, has given him important papers for safe keeping. Unknown to Cornelius, his neighbour Isaac Boxtel , a rival tulipomaniac. has been watching events in the house through a telescope.

Although it has its full share of the excitement and suspense that one tends to associate with Dumas, this is a story that touches the heart as much as it tingles the nerve. It is also unique in that the principal character is not a man or a woman, but a flower - the fateful Tulipa Negra.