London, 1920s. Robert Mawdesley learns that his eccentric ancestor, Simon Mawdesley, left a letter with his bank, with instructions that it should be opened at the stroke of midnight on 4th September that year in the presence of an attorney, a representative of the bank and a 'distinguished physician learned in Indian occultism'. Mawdesley assembles the appropriate people, and at the prescribed time, opens and reads the note. It relates a tale of Venice in the early 18th century...

Count Vittorio Dandolo unsuccessfully courts Leonora, Countess Almoro. Eventually she receives him, but insists it is a friend, not a lover, that she needs. Meanwhile, Leonora's libertine husband is having an illicit with a courtesan, La Foscolina. When the Gazetta Veneziana, whose editor receives his information from the Count's maid, prints gossip about the affair, Almoro has the editor kidnapped and orders his hands to be crushed.

Vittorio spies Almoro and La Foscolina together and challenges his rival to a duel, but La Foscolina intervenes.

An English scientist living nearby is attacked by a thief and left for dead. He is found by Vittorio, who assists him into his home. Searching for something to revive the scientist, Vittorio comes upon an occupied coffin, but says nothing. Reviving, the scientist introduces himself as Simon Mawdesley. He and Vittorio become firm friends. One day, Vittorio is stunned at the appearance of a manservant - the same man he saw in the coffin. Mawdesley explains his research into extending life by inducing suspended animation.

Leonora confides her unhappiness to Vittorio and expresses her fear that she might lose her beloved young son. Vittorio promises to find a way to save them both if she will declare her love for him. Meanwhile they see each other fleetingly.

At the behest of the vengeful editor, Almoro's maid poisons her master's wine. But Almoro realises and, finding a note from Vittorio on her, assumes he is responsible. He confronts Leonora and challenges her to drink. Certain Vittorio would not do such a thing, she drinks. Vittorio, summoned by the frantic maid, arrives in time to see his beloved collapse. In his anger, he strikes Almoro down and escapes.

Desparate, Vittorio runs to Mawdesley, who agrees to help him by making him his 'greatest experiment'. At Vittorio's request, Mawdesley gives him a drug in case the experiment fails. He places a letter in the coffin for when Vittorio revives.

1920s. Robert Mawdesley and the physician travel to Venice to try and revive Vittorio. They uncover the crypt and perform the ritual, then leave, fearing that seeing them might shock the waking Vittorio. Reviving, Vittorio, believing he has slept only a short time, heads for the Almoro palazzo, where the count's family remain, although their fortunes have declined. He encounters young Genevra Almoro, and immediately takes her for Leonora. When Mawdesley and the physician arrive, however, he realises the truth.

Mawdesley introduces Vittorio to 20th Century life. He shows him Simon Mawdesley's letter, which warns that life might seem colourless, and that he might find himself utterly without desire. The letter reminds him of the drug in his pocket.

Vittorio befriends Genevra, and is shocked when he first encounters her unworthy suitor, her cousin Count Gardi-Almoro, who he initially takes for his old adversary.

Hoping marriage to Genevra might cure his lack of zest, Vittorio proposes and she accepts. But when he embraces her he realises his passion has not returned. The marriage is miserable, and Genevra renews her acquaintance with Gardi-Almoro.

Gardi-Almoro conveys his love for Genevra, and suggests an illicit meeting. Vittorio follows him and confronts him with his inappropriate behaviour, but the fight is stopped by the arrival of Genevra. Challenged by her husband, she insists that she never loved Gardi-Almoro, but bemoans Vittorio's coldness.

Realising the hopelessness of his situation, Vittorio dons his 18th century clothes and takes the drug, after writing a letter to Genevra. Alerted to Vittorio's strange behaviour, Leonora enters his room to find him convulsed. His suffering reawakens her love for him. In his final moments, he manages one last, passionate kiss, then dies. Genevra collapses at his feet in grief.