During an intense fight scene in the movie Tenet being screened in a Shaw Theatres hall at Nex mall, the audience could hear a strange crumbling noise coming from above.

Some of them thought it was part of the sound effects – but for two moviegoers, the reality was a painful experience.

The two women were injured when a large ventilation duct was dislodged from the ceiling and landed close to them.

One of them escaped with minor bruises, but the other was badly hurt after a big chunk of plaster debris hit her head.

Both women were taken to Tan Tock Seng Hospital (TTSH), where the injured woman remained warded yesterday after the other woman was discharged.

A TTSH spokesman yesterday declined to reveal the woman's condition and injuries, citing patient confidentiality.

The incident occurred at about 4.45pm on Sunday when the ventilation duct, which reportedly weighed at least 100kg, fell from the ceiling in Shaw Theatres' Hall 6 on the fourth level at Nex mall in Upper Serangoon.

The duct, which Chinese evening daily Lianhe Wanbao reported was about 1.2 metres long, crashed into three rows of seats, sending debris flying everywhere and patrons fleeing for the exits.

But full-time national serviceman Sebastian Koo, 19, stayed behind to tend to the injured woman, who sat behind him and his four male buddies.

He told The Straits Times that he was alarmed to see a large chunk of plaster debris had struck her head.

Mr Koo, who is trained in first aid as part of the Red Cross Youth in secondary school, said: "She was lying in a fetal position and clutching her head, but I don't think I saw any bleeding and she was conscious."

The cinema staff offered her a cup of water and she could soon sit up, he added.

"I asked if she was okay and she was able to respond to my questions, though she complained that her head was very painful."

One of his friends called the Singapore Civil Defence Force, while another two helped ensure everyone evacuated the hall and that no one was trapped underneath the debris.

Mr Koo said: "It was very instinctual for me to quickly snap out of my state of shock and make sure everyone was okay. I was so afraid that I was not able to help those around me," he said.

Recalling the incident, he told ST he had heard the noise from above during the movie.

"It was in the middle of an intense fight scene, so I thought it was just an extremely realistic sound effect," he said.

But dust soon began to fall from the ceiling as it cracked open, followed by the ventilation duct falling to the floor with plaster debris from the ceiling.

Mr Koo said it was fortunate that the duct, which was large enough to cover at least four seats, did not hit anyone, though two of his friends had minor scrapes from debris hitting their heads.

Some of the 20 patrons who were watching Christopher Nolan's latest mind-bending thriller also had scrapes from being hit by the debris.

About 50 patrons from other halls were also evacuated by Shaw staff, who noted their contact details for ticket refunds.

A Shaw Theatres spokesman said its staff attended to injured patrons until paramedics arrived.

Apologising for the incident, the spokesman wished the injured patrons a speedy recovery.

"Our priority at this time is to provide support to our injured patrons and their families. We are in contact with one family and have reached out to the other," he said.

The cinemas will be closed until further notice pending investigations.

Meanwhile, the incident has made news overseas, including in the Hollywood trade magazine Variety.