And it was not so surprising, really, that she didn’t recognize him. It wasn’t just that he had put on false whiskers. He had put himself into an entirely different class. He had donned the uniform of a working man. Of course she wouldn’t recognize him. She barely looked at him. He had become invisible to her.

Naturally he knew who she was and so stared at her with a dangerous fixity that was underlined by a reprise of his theme from the band.

She only noticed the ticket collector – with a vaguely troubled frown – when he refused to move on, long after he had examined their tickets. It took an interjection from her husband to prompt him to leave them in peace.

By the time the police found the real ticket inspector in the mail carriage, stripped to his underclothes, trussed up and gagged, his imposter had long since jumped from the moving train.

The couple honeymooned in Venice. Inevitably there was a scene on a gondola, and even more inevitably, the extravagantly mustachioed gondolier turned out to be none other than the former cavalry officer. If there were titters at the implausibility of this, Quinn did not hear them. It occurred to Quinn that, by virtue of the murders he had committed, the character had acquired a kind of mythical status, becoming almost a supernatural being, like a Hindu avatar. Or perhaps these scenes were merely the mirror images of those in which the murderer was haunted by the eyes of his beloved? That is to say, the former cavalry officer was not really there, it was simply that Eloise’s character saw him wherever she looked.

When the couple dined in a restaurant, the waiter bringing them their food was the former cavalry officer. He stood over them, curling his lip, as they fed each other ice cream from tall glasses with long spoons. The friendly priest who pointed out the mosaics in St Mark’s Basilica – Quinn knew him immediately. The flower vendor in the Piazza di San Marco, the Carabiniere lurking on the Rialto Bridge, the attendant who showed them to their box at the theatre, the tenor on the stage, the dancer in the beaked Medico della Peste mask who led the masquerade through the midnight alleyways, the night porter who greeted them on their return to their hotel … they were all him. Of that Quinn had no doubt.

So it was clear that the psychological explanation was the one that the film was forcing on the audience. Although she was on honeymoon with the Count of Somewhere or Other, Eloise’s character could not get the image of her former lover out of her mind. But just when Quinn persuaded himself to be satisfied with this explanation, he remembered the trussed-up guard on the express train.

On the surface, Eloise remained untroubled by these visions, if indeed she was conscious of them. She dined and danced and visited the sights with her bridegroom. On her lips was always the happy smile of a new bride. But her eyes were a different matter. There was no doubt now that her eyes had a haunted quality to them.

The director again chose to present an extended close-up of those eyes, just as he had done at the beginning of the film. As before, the camera’s viewpoint moved back to show her whole face. In contrast to the earlier sequence, a hand – her husband’s, presumably – came into the frame, this time though to caress her cheek with a loving touch. Tellingly, Eloise flinched away.

A dramatic flurry of high, discordant intervals from the band underlined the significance of this gesture. Her husband’s expression was wounded. To repair the damage, she grasped his hand and held it to her cheek, her eyes closed longingly. She was wishing herself into love for this man. But Quinn knew that the truth was concealed behind those eyelids. Her eyes still desired the disgraced cavalry officer. This was clear the next time she opened them and the director once again treated the audience to an extreme, overwhelming close-up. Eyes were not meant to be seen so large, nor looked into for so long. Quinn became aware of how long she had held them open without blinking. He began to feel tense and uncomfortable. It was inhuman, almost cruel, to force him to continue looking into those eyes.

The eyes themselves, once objects of beauty, became objects of terror. It was not just the melodramatic music that suggested this idea.

There was horror in their gaze, a dawning realization of the tragedy and abasement that lay ahead. A despair so complete that it took away hope from all who gazed into them.

By another of his clever camera tricks, the director revealed that the isolated eyes were looking down from a cracked plaster ceiling at the former cavalry officer, who was stretched out on the grubby mattress of an iron-framed bed. A metallic object glinted in his hands. Quinn recognized it as one of the long spoons with which the couple had eaten ice cream. And so, once again, he had to reassess his interpretation of what he was seeing. Was he to take it that the cavalry officer really had been the waiter in the restaurant, and therefore all the other manifestations?

The next scene showed the couple asleep in a hotel suite. The large window was open on a night dominated by an enormous full moon. Eloise stirred and woke as a shadowy figure appeared silhouetted against the moon, climbing in through the window.

She did not cry out. She knew it was him. The man she had seen wherever she looked. He had come for her.

She rose from the bed; they looked into each other’s eyes. And then threw themselves at each other.

As they kissed, the Count of Somewhere or Other woke. He leapt from the bed with a cry, which the band did its best to convey musically. A struggle ensued between the count and the former cavalry officer. But the latter had come armed. A close-up showed a stiletto blade sinking into the count’s soft flank. He fell to the floor.

The lovers confronted one another. Once again, Eloise’s eyes held and conveyed the entire meaning of the moment. The horror that had been nascent in them before bloomed into a deep revulsion. The former cavalry officer had to see that all was lost. Quinn certainly understood this.

She hated him. There was only hatred in those eyes now.

The next sequence showed the cavalry officer stealing away from the couple’s suite. The band rumbled ominously and then fell silent as the camera returned to the bedroom.

There were now two bodies on the floor. The director went in for his favourite close-up. But where Eloise’s eyes should have been there were two black chasms.

Quinn couldn’t say whether he was one of those who shrieked. But he certainly felt himself bodily leave his seat.

Of course, he had seen worse in reality. But there was something about having such horrors depicted in art that he found more shocking than to discover them in the course of his professional life. Their representation signified some kind of acknowledgement. It opened a door.

If it was only policemen and police surgeons – and the occasional accidental witness – who were obliged to confront such crimes, they could be contained and prevented from contaminating the wider society. It was part of his job to take these things on himself. It was his responsibility. He thought of Sir Edward’s secretary, Miss Latterly. Her horrified reaction to the little he had inadvertently let slip about his work was perfectly proper. This was how the public ought to view such things.

This wallowing in horror and violence, this fascination with gruesome and macabre spectacle, it was unwholesome. It was obscene. More than that, it was dangerous. Who knew where it would lead?

But the film wasn’t over yet. Quinn could not imagine what else, what new horrors, the makers could have in store for the audience. But he sensed the eager anticipation in those around him.

The murderer returned to his lair. It was not clear where this was, whether in Venice, or Vienna. It didn’t matter. It was a psychological place. It existed both as an idea in the murderer’s head, and – now that it had been filmed – as an external reality. It was inside all their heads now.