She went down the ways at Blohm+Voss in Hamburg on 14 May 1927: 27,561 tons, 205.9 metres long, three funnels, and a full-size tennis court behind the third one. On 29 October she sailed for Buenos Aires, briefly the largest and fastest ship on the Hamburg-South America Line's route until an Italian liner took the title a month later. She was named for Cape Arkona, on Rügen. For a decade she was the company's prestige ship. Nobody who sailed on her in those years had reason to remember her for anything else.

The Kriegsmarine took her in 1940, painted her overall grey and moored her at Gotenhafen — now Gdynia — as an accommodation ship. In 1942 she was dressed up again, this time as the Titanic, her funnels and hull partly repainted in White Star colours for the exteriors of the Nazi propaganda film. The production ended badly: its director, Herbert Selpin, was arrested over remarks about Kriegsmarine officers, interrogated by Goebbels, and found hanged in his cell the next morning.

In 1945 she was put back to sea for Operation Hannibal and carried 25,795 soldiers and civilians west from East Prussia, across water thick with mines and Soviet submarines. Her captain, Johannes Gertz, shot himself in his cabin at Copenhagen on 20 February rather than make another crossing. By the end of March her turbines were finished, and she was sent to anchor in Neustadt Bay.

Between 16 and 28 April 1945 the SS emptied Neuengamme concentration camp near Hamburg, along with others, and moved the prisoners towards the coast ahead of the advancing British and Canadian armies. In Lübeck Bay they assembled a flotilla of ships that could no longer sail: the Cap Arcona, the freighter Thielbek, the liner Deutschland, and the small Athen, which ferried people out to them. The prisoners were locked below decks and in the holds, and given neither food nor medical care.

They came from at least thirty countries — Polish, Russian, French, Dutch, Danish, Norwegian, Czechoslovak, Hungarian, Greek, Italian, Belgian, Ukrainian, Belarusian, Latvian, Lithuanian, Estonian, Serbian, Spanish, German, American, Canadian. Around five thousand were put aboard the Cap Arcona, roughly 2,800 aboard the Thielbek. On the evening of 2 May more arrived by barge, mostly women and children from Stutthof and Mittelbau-Dora. The Cap Arcona would take no more, so over eight hundred were sent back to the beach at Neustadt — and on the morning of 3 May about five hundred of them were killed there by their SS guards, machine-gunned in the barges or beaten to death on the sand.

British intelligence had been told. On 2 May, Swedish and Swiss Red Cross officials passed on that large numbers of prisoners were aboard ships anchored in Lübeck Bay, and in Lübeck a representative of the International Committee of the Red Cross told Major-General Philip Roberts that seven to eight thousand people were out there. The information never reached the squadrons.

What the RAF commanders believed was that the ships were being readied to carry senior SS officers to German-held Norway, an escape the Allies knew about from decrypted signals. Hawker Typhoons of No. 83 Group attacked with rockets, bombs and 20 mm cannon, as part of general strikes on Baltic shipping. The Cap Arcona burned and capsized. Prisoners who got over the side found water at about seven degrees, and some of the aircraft strafed the men swimming in it. One pilot, Allan Wyse of 193 Squadron, said long afterwards: “We used our cannon fire at the chaps in the water... Horrible thing, but we were told to do it and we did it.” Of the roughly five thousand prisoners aboard the Cap Arcona, some 350 survived; about fifty came off the Thielbek. Every figure is an estimate, and totals for the day run to around seven thousand dead.

British reconnaissance troops reached Neustadt that afternoon, watched the ships burning offshore, and pulled a few emaciated survivors off the beach. Most of what they found there were the bodies of the women and children killed that morning. For weeks the dead came in on the tide and were buried in mass graves along the coast, and those graves are the clearest map of the thing now: 810 at the Waldfriedhof in Timmendorfer Strand, 407 at Grevesmühlen, 113 at Niendorf, 100 in the Jewish cemetery at Neustadt, 91 in the churchyard at Grömitz.

Some who lived went on to long lives — Erwin Geschonneck, later one of East Germany's best-known actors, who eventually told the story on film; the Czech theatre director Emil František Burian; Benjamin Jacobs, born Berek Jakubowicz in Dobra, who spent decades afterwards speaking about the Holocaust. It is among the largest single losses of life at sea in the Second World War, and it happened within sight of the shore, on the last full day of the war in northern Germany.