Full spoilers follow.
The setup. Kang Min-ho's father ran a large company; after his death, Min-ho's uncle took control of it. Min-ho grows into a cut-throat investor whose entire professional life is a weapon aimed at reclaiming that firm and punishing the man who took it. He is also dying — a heart condition gives him very little time — which is why his revenge campaign runs at such a merciless pace in the early episodes.
The transplant. A donor heart arrives, and the operation succeeds. What follows is the show's real premise: the cold-hearted Min-ho begins to feel warmth, softens toward the people he would previously have crushed, and falls for Kim Soon-jung — who happens to be his uncle's secretary, positioned squarely inside enemy territory.
The central twist. Min-ho pays to have his donor identified. The donor is Ma Dong-wook — Soon-jung's brother. This single fact retroactively reframes the entire romance: the woman he loves is the sister of the man whose heart is keeping him alive, and the 'tragic link' the leads share turns out to be the organ in his chest. The revelation is corroborated by feeling as much as by evidence: Soon-jung senses something of Dong-wook in Min-ho, and Lee Joon-hee reports that looking at Min-ho practically feels like staring at Dong-wook himself.
The explanation the show half-offers. Doctor Jo comes to believe Min-ho may be in the one percent of transplant cases exhibiting cellular memory syndrome — the theory that an organ recipient acquires characteristics of the donor. The drama deploys this as a legitimising frame rather than a verdict, and continues to pose its question openly: is the change the heart, or the people? Min-ho's own overdue self-reflection begins here, and his questioning of whether his feelings for Soon-jung are genuinely his own is the natural consequence.
The endgame. The corporate conflict with the uncle resolves, and Min-ho's health threats return to menace the final stretch — but the finale delivers a full happy ending. Soon-jung and Min-ho get their happily ever after, sealed by a much-loved closing beat in which the two take turns kissing each other depending on which direction passers-by are approaching from. The secondary couple nicknamed Wendy and Tinkerbell are also paired off happily.
What the finale leaves open. The most-noted gap is medical: there is no proper explanation for how Min-ho survives the dire health threats stacked against him and emerges healthy and smiling. Read generously, this is the healing melodrama keeping its promise — the story was always about earning the right to an ordinary morning, not about surgical plausibility. Read critically, the drama walks away from the mortality that gave its first half urgency. Either way, the deepest meaning only lands in retrospect: every warm impulse the audience enjoyed in the middle episodes was, unknowingly, a dead man's sister being courted by his heart — which is why the show's happiest ending is also, quietly, an act of mourning completed.