Under a Dark Sun, set against the backdrop of the Mediterranean sun, focuses on Alba, who faces a murder charge after Arnaud Lasserre, owner of the flower farm where her mother once worked, dies under mysterious circumstances.
The mystery in the French thriller mini-series centers on the fact that Arnaud's will agreement has Alba's name on it, offering a part of his estate to her. In Under a Dark Sun, Alba then finds out that she is the daughter of Arnaud's son Mathieu, who once had an affair with her mother Nadia, and got her pregnant. Nadia was then sent away by Arnaud to raise Alba alone. After the patriarch's death, Alba was framed for his death as a calculated move to remove her from the estate affairs.
An important narrative track in Under a Dark Sun also involves Manon, Arnaud’s granddaughter and Mathieu’s legitimate daughter. It is revealed that it was Manon who killed Arnaud after coming to know how he had treated her mother, Josaphine. She then pretends to be Alba's lawyer, to influence the case in a way that she is not exposed. By the end, however, the plot ensures Alba and Manon are aware of a terrible secret about each other. So, they reach an agreement to keep quiet. Read on to understand the complete ending of Under a Dark Sun.
Here is how Under a Dark Sun ends
As the season concludes, Alba finally unearths the truth behind her origin. Years after Nadia was sent away from the estate, she wrote to Arnaud telling him to insert Alba's name in his will. This is how Nadia secured her daughter's future.
Alba then finds out that Arnaud's son Mathieu is her father, as he once had an affair with Nadia when she was a worker on the farm. Arnaud then threw Nadia out of the estate, but the family scandals don't end here. When Josephine, Mathieu's lawfully wedded wife, finds out about his affair with Nadia, she is sent to a psychiatric institution to prevent her from saying anything that might damage the family name.
It is then that Alba is framed for killing Arnaud, and Manon comes forward, apparently to defend her. Manon was just trying to influence the case and protect herself. After Arnaud's death, Beatrice, his complicit wife, was also sent to the same psychiatric institution, and there Manon finds Josephine and decides to take revenge, by killing her grandfather for what he did to her mother. Also, she realises Alba is her half-sister.
Who kidnapped Alba's son Leo in Under a Dark Sun?

Alba's son, Leo, was kidnapped, and the only thing he remembers about his kidnapper is a tattoo. While practicing archery on the farm, Leo sees Hadrien, Manon's brother. He panics because Hadrien has the same tattoo. Leo thinks Hadrien was his kidnapper, and ends up killing him. This is a major reason why Manon decides to frame Alba and misguide her case, by pretending to fight her case. It turns out that Leo had mistakenly killed Hadrien, while it was Manon who had kidnapped him. Leo got confused because Manon, too, had the same tattoo as her brother.
Alba and Manon, when they get to know about each other's past, decide to keep mum about each other's crimes. They agree to frame Hadrien for Arnaud's death, saving both of them from murder charges.
In the final scenes of Under a Dark Sun, Alba and Leo are shown to be safe. However, the estate's fate remains ambiguous — whether it will be sold or preserved isn't clear. The grip of generational power has been broken, and Alba is shown walking away with her son. She is free of all accusation and has also learned the truth about her origin.