On 17 February last year, a young Hong Kong couple’s holiday in Taiwan went terribly wrong.

The night before they were due to leave, the couple bought a large pink suitcase at one of Taipei’s popular night markets.

Their hotel’s CCTV footage showed the couple returning to their room with the suitcase. That was the last time the 20-year-old woman, Poon Hiu-wing, was seen alive.

The next morning, the footage showed her boyfriend Chan Tong-kai, who was 19 at the time, taking the suitcase, along with other luggage, out of the room to check out.

There was no sign of Ms Poon.

Chan was later seen pushing the suitcase through Taipei’s busy subway stations. He took a flight back to Hong Kong that evening, alone.

It was a month later, after Ms Poon’s father came to Taiwan to report her missing, that her body was found hidden in bushes, about 20 metres from a popular riverside trail in neighbouring New Taipei City.

“There was a stench, but no one thought there would be a dead body there,” said Mr Chou, a long-time local resident.

“Sometimes as many as 10,000 dead fish would float up in the river here and it would smell bad, so everyone thought it was just the smell of dead fish.”

How did the case trigger the Hong Kong protests?

The Hong Kong authorities used Taiwan’s request for Chan to be extradited as a reason for proposing a controversial bill to allow such extraditions.

The bill would have allowed criminal suspects to be sent to Taiwan, as well as mainland China.

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The former British colony does not yet have an extradition agreement with Taiwan or mainland China. But many Hong Kongers took to the streets in protest at the plans.

Given China’s human rights violations and heavily government-controlled judicial system, they feared the bill would harm Hong Kong’s judicial autonomy, established under the “one country, two systems” principle.

Taiwan also strongly objected, accusing the Hong Kong government of using its request for Chan’s extradition as an excuse to ram through the bill.

The bill has since been withdrawn due to widespread protests, but Chan’s case is still in limbo.

What happen on the night in question?

According to Hong Kong’s High Court document on the case, Chan and Ms Poon met in 2017 and became lovers a month later.

Ms Poon was five weeks’ pregnant when, in December, Chan arranged a trip to Taiwan for the couple, paying for airplane tickets and hotel accommodation.

After returning to the hotel on 16 February, they argued about how to pack the suitcase. They later made up but in the very early hours of 17 February, they argued again.

It was during that argument that Ms Poon told Chan the baby in her womb was conceived with her former boyfriend. Chan said Ms Poon then showed him a video of her having sex with another man.

Feeling angry, he smashed her head against the wall and strangled her from behind with both hands. They struggled on the floor for about 10 minutes until she was dead.

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He then stuffed her body into the pink suitcase and packed her belongings into four plastic bags.

In the morning, Chan threw away Ms Poon’s belongings in bins near the hotel and checked out, taking the suitcase with Ms Poon’s body in it.

He took the subway for about 40 minutes to Zhuwei station, dragged the suitcase along the trail by the Danshui river, found some tall bushes off the path, took the body out and dumped it there.

He tossed the suitcase elsewhere, but kept her ATM card, digital camera and iPhone.

Before leaving Taipei, he used her ATM card to withdraw TWD20,000 (£507, $653) and after returning to Hong Kong, he used her card to withdraw money three times, totalling HK$19,200 (£1,900, $2,450).

How was the crime uncovered?

Chan was arrested on 13 March by Hong Kong police, two days after her parents reported her missing. He was later charged with four counts of dealing with proceeds from a crime, commonly called money laundering.

During the investigation, he confessed that he had killed Ms Poon and revealed where he had dumped the body, according to the court document.

Liu Guan-wu, a Zhuwei police officer at the time, said he and around 30 other officers found the body after a three-hour search.

“We were looking for the suitcase initially, because we thought the body was still inside the suitcase,” said Mr Liu.

“But when we found her, it was just her body there. He didn’t dismember her; he simply curved her body to stuff it inside the suitcase. She was not a tall girl and she was skinny.

“The body was decomposed. There were no signs of injury.”

In Hong Kong, Chan pleaded guilty to money laundering and was sentenced to 29 months in prison, but the High Court reduced the sentence by a third.

On 18 October this year, days before Chan’s release, the Hong Kong government released a statement about his “alleged offence” in Taiwan.

“The courts of Hong Kong have no jurisdiction over it,” the statement said.

“Neither do the local authorities have any ground to extend Chan’s detention or pursue the offence that he was alleged to have committed in Taiwan.”

Why does no one want him?

The case has revealed the conflicted relations between Taiwan and Hong Kong – as well as the island and mainland China, which considers Taiwan a province that must be reunified one day.

It also highlights the unnatural state of affairs among the three sides – places so close to each other, but with no extradition or judicial cooperation agreements.

The long dispute over Taiwan’s sovereignty – whether it’s a part of China or not – makes forming such agreements, usually reached between countries, a very sensitive matter.

On Wednesday, the day he was released from Hong Kong’s jail, Chan bowed deeply before a large crowd of TV cameras and journalists, apologised to Ms Poon’s family, expressed hopes that she would rest in peace, and asked society for forgiveness.

But the bickering continued between Taiwan and Hong Kong.

Taiwan’s authorities initially refused to accept Chan’s surrender, even though they issued an arrest warrant for him.

They suspect the whole thing was orchestrated by Beijing to treat Taiwan as a part of China and had insisted Hong Kong first negotiate a judicial assistance agreement before handing over Chan.

Hong Kong’s authorities dismissed that as “nonsense” and urged Taiwan to take a suspect willing to surrender himself, so as to give justice to Ms Poon and her family.

Police officer Mr Liu said he’s handled other murders, but never one like this, not least because of the political battles involved.

“I hope the case can be dealt with quickly,” he said. “It must be really sad for her parents.”

Perhaps Chan is also eager to find closure.

“For my impulse, for the wrongful act I’ve committed, I’m willing to surrender and go back to Taiwan to face prison sentence and face trial,” he told the media.

“To society and Hong Kong people, I can only say I’m sorry. I hope everyone can forgive me. I hope everyone can give me a chance to be a new person and give me a chance to give back to society.”